Allow custom formatting for TextField

As I discussed in #262, this adds a `formatText` method onto `TextField`. It's super simple. 

I wrote a little repository to show what this can do [here](https://github.com/johnfn/RedGreenText). The meat is [this](https://github.com/johnfn/RedGreenText/blob/master/RedGreenText.as) file, which overrides `TextField` to render the text in a cute way.