allow assigning variables to tasks at run time via CLI

using a similar syntax than setting env variables to command in bash,
but used right after the task:

```bash
task print MESSAGE=Hello!
```

closes #33

args/args_test.go

@@ -0,0 +1,70 @@
+package args_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/go-task/task"
+	"github.com/go-task/task/args"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestArgs(t *testing.T) {
+	tests := []struct {
+		Args     []string
+		Expected []task.Call
+		Err      error
+	}{
+		{
+			Args: []string{"task-a", "task-b", "task-c"},
+			Expected: []task.Call{
+				{Task: "task-a"},
+				{Task: "task-b"},
+				{Task: "task-c"},
+			},
+		},
+		{
+			Args: []string{"task-a", "FOO=bar", "task-b", "task-c", "BAR=baz", "BAZ=foo"},
+			Expected: []task.Call{
+				{
+					Task: "task-a",
+					Vars: task.Vars{
+						"FOO": task.Var{Static: "bar"},
+					},
+				},
+				{Task: "task-b"},
+				{
+					Task: "task-c",
+					Vars: task.Vars{
+						"BAR": task.Var{Static: "baz"},
+						"BAZ": task.Var{Static: "foo"},
+					},
+				},
+			},
+		},
+		{
+			Args: []string{"task-a", "CONTENT=with some spaces"},
+			Expected: []task.Call{
+				{
+					Task: "task-a",
+					Vars: task.Vars{
+						"CONTENT": task.Var{Static: "with some spaces"},
+					},
+				},
+			},
+		},
+		{
+			Args: []string{"FOO=bar", "task-a"},
+			Err:  args.ErrVariableWithoutTask,
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("TestArgs%d", i+1), func(t *testing.T) {
+			calls, err := args.Parse(test.Args...)
+			assert.Equal(t, test.Err, err)
+			assert.Equal(t, test.Expected, calls)
+		})
+	}
+}