As spotted by the extended linter.

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.

Most of the basic stuff is working, some harder stuff like recursive
macro declarations or shell expansion isn't yet supported. Note that the
spec mode would benefit from a bash mode, as RPM spec files include
shell scripts.