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

task_test.go

@@ -61,8 +61,10 @@ func TestVars(t *testing.T) {
 	}{
 		{"foo.txt", "foo"},
 		{"bar.txt", "bar"},
+		{"baz.txt", "baz"},
 		{"foo2.txt", "foo2"},
 		{"bar2.txt", "bar2"},
+		{"baz2.txt", "baz2"},
 		{"equal.txt", "foo=bar"},
 	}