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.

testdata/vars/Taskfile.yml

@@ -7,17 +7,49 @@ hello:
     - echo {{.FOO}} > foo.txt
     - echo {{.BAR}} > bar.txt
     - echo {{.BAZ}} > baz.txt
+    - echo '{{.TMPL_FOO}}' > tmpl_foo.txt
+    - echo '{{.TMPL_BAR}}' > tmpl_bar.txt
+    - echo '{{.TMPL_FOO2}}' > tmpl_foo2.txt
+    - echo '{{.TMPL_BAR2}}' > tmpl_bar2.txt
+    - echo '{{.SHTMPL_FOO}}' > shtmpl_foo.txt
+    - echo '{{.SHTMPL_FOO2}}' > shtmpl_foo2.txt
+    - echo '{{.NESTEDTMPL_FOO}}' > nestedtmpl_foo.txt
+    - echo '{{.NESTEDTMPL_FOO2}}' > nestedtmpl_foo2.txt
     - echo {{.FOO2}} > foo2.txt
     - echo {{.BAR2}} > bar2.txt
     - echo {{.BAZ2}} > baz2.txt
+    - echo '{{.TMPL2_FOO}}' > tmpl2_foo.txt
+    - echo '{{.TMPL2_BAR}}' > tmpl2_bar.txt
+    - echo '{{.TMPL2_FOO2}}' > tmpl2_foo2.txt
+    - echo '{{.TMPL2_BAR2}}' > tmpl2_bar2.txt
+    - echo '{{.SHTMPL2_FOO}}' > shtmpl2_foo.txt
+    - echo '{{.SHTMPL2_FOO2}}' > shtmpl2_foo2.txt
+    - echo '{{.NESTEDTMPL2_FOO2}}' > nestedtmpl2_foo2.txt
     - echo {{.EQUAL}} > equal.txt
+    - echo {{.OVERRIDE}} > override.txt
   vars:
     FOO: foo
     BAR: $echo bar
     BAZ:
       sh: echo baz
+    TMPL_FOO: "{{.FOO}}"
+    TMPL_BAR: "{{.BAR}}"
+    TMPL_FOO2: "{{.FOO2}}"
+    TMPL_BAR2: "{{.BAR2}}"
+    SHTMPL_FOO:
+      sh: "echo '{{.FOO}}'"
+    SHTMPL_FOO2:
+      sh: "echo '{{.FOO2}}'"
+    NESTEDTMPL_FOO: "{{.TMPL_FOO}}"
+    NESTEDTMPL_FOO2: "{{.TMPL2_FOO2}}"
+    OVERRIDE: "bar"
 
 set-equal:
   set: EQUAL
   cmds:
     - echo foo=bar
+
+invalid-var-tmpl:
+  vars:
+    CHARS: "abcd"
+    INVALID: "{{range .CHARS}}no end"

testdata/vars/Taskvars.yml

@@ -2,3 +2,11 @@ FOO2: foo2
 BAR2: $echo bar2
 BAZ2:
   sh: echo baz2
+TMPL2_FOO: "{{.FOO}}"
+TMPL2_BAR: "{{.BAR}}"
+TMPL2_FOO2: "{{.FOO2}}"
+TMPL2_BAR2: "{{.BAR2}}"
+SHTMPL2_FOO2:
+  sh: "echo '{{.FOO2}}'"
+NESTEDTMPL2_FOO2: "{{.TMPL2_FOO2}}"
+OVERRIDE: "foo"