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/params/Taskfile.yml

@@ -1,7 +1,8 @@
 default:
   vars:
     SPANISH: ¡Holla mundo!
-    PORTUGUESE: "{{.PORTUGUESE}}"
+    PORTUGUESE: "{{.PORTUGUESE_HELLO_WORLD}}"
+    GERMAN: "Welt!"
   deps:
     - task: write-file
       vars: {CONTENT: Dependence1, FILE: dep1.txt}
@@ -20,7 +21,17 @@ default:
       vars: {CONTENT: "{{.SPANISH}}", FILE: spanish.txt}
     - task: write-file
       vars: {CONTENT: "{{.PORTUGUESE}}", FILE: portuguese.txt}
+    - task: write-file
+      vars: {CONTENT: "{{.GERMAN}}", FILE: german.txt}
+    - task: non-default
 
 write-file:
   cmds:
     - echo {{.CONTENT}} > {{.FILE}}
+
+non-default:
+  vars:
+    PORTUGUESE: "{{.PORTUGUESE_HELLO_WORLD}}"
+  cmds:
+    - task: write-file
+      vars: {CONTENT: "{{.PORTUGUESE}}", FILE: portuguese2.txt}

testdata/params/Taskvars.yml

@@ -1 +1,2 @@
-PORTUGUESE: Olá, mundo!
+PORTUGUESE_HELLO_WORLD: Olá, mundo!
+GERMAN: "Hello"