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

Add CodeMirror.defineOption function for defining new options.

Add findMatchingBracket to the methods exported by this extension.

Closes #746

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

Issue #725

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.