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.
The main barrier I see to combining them is that attributed text should not have a `set_color` method, and I couldn't see how to combine into a single class without having the `set_color` method just silently do nothing if `attributed=True`.  I could set it up so that `Text` inherits from `AttrText`, but all that does is save duplicating the `draw` method, it doesn't prevent the duplicated code in  `__init__` (or if it could, I haven't figured out how).