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.
Even though it's not backward-compatible, maybe we should make this default to `True`. Will it only break things that use `{}`? If so, we might be safe so far.