Reintroduce template evaluation in variables

With a recent commit, template evaluation for variables in tasks got
broken. This reindroudces temmplate evaluation in taks, and resolves
a series of issues that where previouisly present on master, such as:

- Taskvars did not get evaluated as templates.
- Taskvars would, in contrast to the documentation, _override_ task
  variables for the taks called directly via `Executor.Run(args
  ...string)`. This caused different behaviour in the "default" task
  v.s. other tasks.

This commit ensures:
 - Priority order for variables is now according to the documentation,
   also for the "default" task.
 - Variables gets resolved in a particular order to ensure logical
   access to varaibles on template compile time, and that template
   compilation finds place _before_ resolution of dynamic variables.

This change also allows the following to work:

    task:
      vars:
        A: "52"
        B: "{{.A}}"

However, the following will always replace C with the uncompiled
`{{.A}}`:

    task:
      vars:
        A: "52"
        C: "{{.B}}"
        B: "{{.A}}"

Several tests have also been added to prevent this feature from breaking
again. This should hopefully finally resolve issue #40.

task_test.go

@@ -14,6 +14,121 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+// fileContentTest provides a basic reusable test-case for running a Taskfile
+// and inspect generated files.
+type fileContentTest struct {
+	Dir       string
+	Target    string
+	TrimSpace bool
+	Files     map[string]string
+}
+
+func (fct fileContentTest) name(file string) string {
+	return fmt.Sprintf("target=%q,file=%q", fct.Target, file)
+}
+
+func (fct fileContentTest) Run(t *testing.T) {
+	for f := range fct.Files {
+		_ = os.Remove(filepath.Join(fct.Dir, f))
+	}
+
+	e := &task.Executor{
+		Dir:    fct.Dir,
+		Stdout: ioutil.Discard,
+		Stderr: ioutil.Discard,
+	}
+	assert.NoError(t, e.ReadTaskfile(), "e.ReadTaskfile()")
+	assert.NoError(t, e.Run(fct.Target), "e.Run(target)")
+
+	for name, expectContent := range fct.Files {
+		t.Run(fct.name(name), func(t *testing.T) {
+			b, err := ioutil.ReadFile(filepath.Join(fct.Dir, name))
+			assert.NoError(t, err, "Error reading file")
+			s := string(b)
+			if fct.TrimSpace {
+				s = strings.TrimSpace(s)
+			}
+			assert.Equal(t, expectContent, s, "unexpected file content")
+		})
+	}
+
+}
+
+func TestVars(t *testing.T) {
+	tt := fileContentTest{
+		Dir:       "testdata/vars",
+		Target:    "default",
+		TrimSpace: true,
+		Files: map[string]string{
+			// hello task:
+			"foo.txt":              "foo",
+			"bar.txt":              "bar",
+			"baz.txt":              "baz",
+			"tmpl_foo.txt":         "foo",
+			"tmpl_bar.txt":         "<no value>",
+			"tmpl_foo2.txt":        "foo2",
+			"tmpl_bar2.txt":        "bar2",
+			"shtmpl_foo.txt":       "foo",
+			"shtmpl_foo2.txt":      "foo2",
+			"nestedtmpl_foo.txt":   "{{.FOO}}",
+			"nestedtmpl_foo2.txt":  "foo2",
+			"foo2.txt":             "foo2",
+			"bar2.txt":             "bar2",
+			"baz2.txt":             "baz2",
+			"tmpl2_foo.txt":        "<no value>",
+			"tmpl2_foo2.txt":       "foo2",
+			"tmpl2_bar.txt":        "<no value>",
+			"tmpl2_bar2.txt":       "<no value>",
+			"shtmpl2_foo.txt":      "<no value>",
+			"shtmpl2_foo2.txt":     "foo2",
+			"nestedtmpl2_foo2.txt": "{{.FOO2}}",
+			"equal.txt":            "foo=bar",
+			"override.txt":         "bar",
+		},
+	}
+	tt.Run(t)
+	// Ensure identical results when running hello task directly.
+	tt.Target = "hello"
+	tt.Run(t)
+}
+
+func TestVarsInvalidTmpl(t *testing.T) {
+	const (
+		dir         = "testdata/vars"
+		target      = "invalid-var-tmpl"
+		expectError = "template: :1: unexpected EOF"
+	)
+
+	e := &task.Executor{
+		Dir:    dir,
+		Stdout: ioutil.Discard,
+		Stderr: ioutil.Discard,
+	}
+	assert.NoError(t, e.ReadTaskfile(), "e.ReadTaskfile()")
+	assert.EqualError(t, e.Run(target), expectError, "e.Run(target)")
+}
+
+func TestParams(t *testing.T) {
+	tt := fileContentTest{
+		Dir:       "testdata/params",
+		Target:    "default",
+		TrimSpace: false,
+		Files: map[string]string{
+			"hello.txt":       "Hello\n",
+			"world.txt":       "World\n",
+			"exclamation.txt": "!\n",
+			"dep1.txt":        "Dependence1\n",
+			"dep2.txt":        "Dependence2\n",
+			"spanish.txt":     "¡Holla mundo!\n",
+			"spanish-dep.txt": "¡Holla dependencia!\n",
+			"portuguese.txt":  "Olá, mundo!\n",
+			"portuguese2.txt": "Olá, mundo!\n",
+			"german.txt":      "Welt!\n",
+		},
+	}
+	tt.Run(t)
+}
+
 func TestDeps(t *testing.T) {
 	const dir = "testdata/deps"
 
@@ -52,48 +167,6 @@ func TestDeps(t *testing.T) {
 	}
 }
 
-func TestVars(t *testing.T) {
-	const dir = "testdata/vars"
-
-	files := []struct {
-		file    string
-		content string
-	}{
-		{"foo.txt", "foo"},
-		{"bar.txt", "bar"},
-		{"baz.txt", "baz"},
-		{"foo2.txt", "foo2"},
-		{"bar2.txt", "bar2"},
-		{"baz2.txt", "baz2"},
-		{"equal.txt", "foo=bar"},
-	}
-
-	for _, f := range files {
-		_ = os.Remove(filepath.Join(dir, f.file))
-	}
-
-	e := &task.Executor{
-		Dir:    dir,
-		Stdout: ioutil.Discard,
-		Stderr: ioutil.Discard,
-	}
-	assert.NoError(t, e.ReadTaskfile())
-	assert.NoError(t, e.Run("default"))
-
-	for _, f := range files {
-		d, err := ioutil.ReadFile(filepath.Join(dir, f.file))
-		if err != nil {
-			t.Errorf("Error reading %s: %v", f.file, err)
-		}
-		s := string(d)
-		s = strings.TrimSpace(s)
-
-		if s != f.content {
-			t.Errorf("File content should be %s but is %s", f.content, s)
-		}
-	}
-}
-
 func TestTaskCall(t *testing.T) {
 	const dir = "testdata/task_call"
 
@@ -225,41 +298,6 @@ func TestInit(t *testing.T) {
 	}
 }
 
-func TestParams(t *testing.T) {
-	const dir = "testdata/params"
-	var files = []struct {
-		file    string
-		content string
-	}{
-		{"hello.txt", "Hello\n"},
-		{"world.txt", "World\n"},
-		{"exclamation.txt", "!\n"},
-		{"dep1.txt", "Dependence1\n"},
-		{"dep2.txt", "Dependence2\n"},
-		{"spanish.txt", "¡Holla mundo!\n"},
-		{"spanish-dep.txt", "¡Holla dependencia!\n"},
-		{"portuguese.txt", "Olá, mundo!\n"},
-	}
-
-	for _, f := range files {
-		_ = os.Remove(filepath.Join(dir, f.file))
-	}
-
-	e := task.Executor{
-		Dir:    dir,
-		Stdout: ioutil.Discard,
-		Stderr: ioutil.Discard,
-	}
-	assert.NoError(t, e.ReadTaskfile())
-	assert.NoError(t, e.Run("default"))
-
-	for _, f := range files {
-		content, err := ioutil.ReadFile(filepath.Join(dir, f.file))
-		assert.NoError(t, err)
-		assert.Equal(t, f.content, string(content))
-	}
-}
-
 func TestCyclicDep(t *testing.T) {
 	const dir = "testdata/cyclic"