CodeMirror 3.1 integration

Commit descriptions pretty much say it all.

Minor cleanups, use an up-to-date CodeMirror 3.1 optimized build instead of several unoptimized css and js files.

Add a setting to allow to disable autoloading of extra js and css files, in case if one has already loaded CodeMirror for other purposes. No auto-detection here, because one has to manually check that he has loaded all the required addons and css files.