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 want to move in that direction because it also saves (DRY) docstring and any new methods we want to add to these classes. It also saves the user from ever having to think about two different classes.

The `set_color` can be handled by adding these two lines:

```
if self._attributed:
    raise RuntimeError(...)
...
```
I'd rather work with one class that can handle everything for now, unless we expect there to be lots of methods that differ for the two use cases.