Add a *.pex parser.

It works with the files created with http://onebyonedesign.com/flash/particleeditor/
Hm... From the parameter alone I would have expected that I have to use `Assets.getText()` myself to pass the file contents. I guess this might be more convenient but not as flexible? i.e. you might also want to use `@:file` instead of `openfl.Assets`.