use YAML for dynamix variable instead of $ prefix

$ prefix still works but is now deprecated

before:

    VAR: $echo var

after:

    VAR:
      sh: echo bar

closes #46

testdata/vars/Taskfile.yml

@@ -6,12 +6,16 @@ hello:
   cmds:
     - echo {{.FOO}} > foo.txt
     - echo {{.BAR}} > bar.txt
+    - echo {{.BAZ}} > baz.txt
     - echo {{.FOO2}} > foo2.txt
     - echo {{.BAR2}} > bar2.txt
+    - echo {{.BAZ2}} > baz2.txt
     - echo {{.EQUAL}} > equal.txt
   vars:
     FOO: foo
     BAR: $echo bar
+    BAZ:
+      sh: echo baz
 
 set-equal:
   set: EQUAL

testdata/vars/Taskvars.yml

@@ -1,2 +1,4 @@
 FOO2: foo2
 BAR2: $echo bar2
+BAZ2:
+  sh: echo baz2