Since v2 is the canonical CodeMirror at this point

It's tiny anyway, no need to force people to include yet another file.

This normalizes the classes used by most of the modes
(reStructuredText and diff didn't fit in), and makes them all
reference a central style sheet (under theme/). It also adds a theme
option that can be set to change the theme, and defines a few
different themes (some based on old mode colors).