Coloring beyond text

It would be nice to color beyond where characters exist. I have some experimental plugins where I want to color a certain area beyond where characters exist.

Here is the gist of it:

https://gist.github.com/twolfson/6191987

This aligns with other requests for coloring of entire lines for diffs, custom `highlight_line` behavior, and custom rulers.

One solution would be a method that takes in vectors and flags, allowing for horizontal or vertical styling.