Support Enums in ValueFormatter.php

[wlkns]

This snippet adds support for string backed PHP Enums.

Example:

enum Animal: string {
   case DOG = 'dog';
   case CAT = 'cat';
   case BIRD = 'bird';
}

[wlkns]

Ping @isublimity :) Can we get this merged or would you like any changes?