Support limited variable interpolation.

Kong supports limited variable interpolation into help strings, enum lists and
default values.

Variables are in the form:

    ${<name>}

Variables are set with the `Vars(map[string]string)` option. Undefined
variable references in the grammar will result in an error at construction
time.

tag.go

@@ -24,7 +24,7 @@ type Tag struct {
 	Short       rune
 	Hidden      bool
 	Sep         rune
-	Enum        map[string]bool
+	Enum        string
 
 	// Storage for all tag keys for arbitrary lookups.
 	items map[string]string
@@ -114,7 +114,6 @@ func parseTag(fv reflect.Value, ft reflect.StructField) *Tag {
 	s, chars := getTagInfo(ft)
 	t := &Tag{
 		items: parseTagItems(s, chars),
-		Enum:  map[string]bool{},
 	}
 	t.Cmd = t.Has("cmd")
 	t.Arg = t.Has("arg")
@@ -149,10 +148,7 @@ func parseTag(fv reflect.Value, ft reflect.StructField) *Tag {
 	if t.PlaceHolder == "" {
 		t.PlaceHolder = strings.ToUpper(dashedString(fv.Type().Name()))
 	}
-	for _, part := range strings.Split(t.Get("enum"), ",") {
-		t.Enum[part] = true
-	}
-
+	t.Enum = t.Get("enum")
 	return t
 }