So that I can put other doc-related stuff there without polluting the
root dir.

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).

- Trailing commas in object literals are not allowed by the spec
  and are handled poorly in some browsers.
- Added var to some variable declarations that were not meant to be globals.