Issue #3682

Issue #3337
Issue #3338

Closes #1133

Since v2 is the canonical CodeMirror at this point

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

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

Closes #239

Using normal replaceRange causes an infinite loop when replacing something
with a string that contains the original string.

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).