ENH: new text class for inline color changes

This would be nice to have for my upcoming experiment: since I'm using color cues, I want to change color inline during the instructional text to make the explanations clearer. I fully acknowledge that pyglet's handling of formatted text is buggy, but it seems to be stable as long as all you change is the color (which is all I need to do) and not the typeface or size.
I have a feeling a lot of the code will be shared between these classes. Could we instead combine them and have an `__init__` `kwarg` to specify `attributed`, and/or a property/setter to specify it?