Added 'active' attribute to FlxButton

Based of findings here: https://github.com/HaxeFlixel/flixel/pull/1227
Would allow setting "active" for an FlxButton in the XML. Helpful if you want to prevent a login button from being pressed until the fields are filled. 
Also updated the documentation (added active and visible in the button attributes list).
Yeah I'm fine with adding this everywhere. Only thing I can think of is some object that depends on a first-frame update to do essential startup stuff, but it's up to the user to use this property wisely in any case.