Issue #1739

Add possibility to attach classes to line wrapper.

Closes #896

The off pun is rather awful, but it does have precedent, and is
more succinct.

This allows multiple handlers to listen for a single event, and makes
the API a bit more uniform.

Issue #725

Closes #409

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.

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).