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.

model.go

@@ -185,10 +185,11 @@ func (n *Node) Path() (out string) {
 
 // A Value is either a flag or a variable positional argument.
 type Value struct {
-	Flag     *Flag
+	Flag     *Flag // Nil if positional argument.
 	Name     string
 	Help     string
 	Default  string
+	Enum     string
 	Mapper   Mapper
 	Tag      *Tag
 	Target   reflect.Value
@@ -198,6 +199,16 @@ type Value struct {
 	Position int    // Position (for positional arguments).
 }
 
+// EnumMap returns a map of the enums in this value.
+func (v *Value) EnumMap() map[string]bool {
+	parts := strings.Split(v.Enum, ",")
+	out := make(map[string]bool, len(parts))
+	for _, part := range parts {
+		out[strings.TrimSpace(part)] = true
+	}
+	return out
+}
+
 // Summary returns a human-readable summary of the value.
 func (v *Value) Summary() string {
 	if v.Flag != nil {