Adds FlxTypeText to flixel-addons, closes #25

Per issue #25. Demo available on request if you'd like to see a (very) basic implementation. Contains all of the features of the original TypeText, but makes many of the fixed capabilities optional (having a prefix, a blinking cursor, cursor blink speed, text speed). The only new feature is the ability to add a slight random variation to typing speed, giving the animation a more natural feel.