Closes #1403

It's better to have it only take effect when in a comment, so that
Enter's regular behavior isn't overridden in other situations.

Closes #1133

Add CodeMirror.defineOption function for defining new options.

Add findMatchingBracket to the methods exported by this extension.

Closes #746

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).