Undo closing bracket if added in token type of null

Re issue: https://groups.google.com/forum/#!topic/codemirror/Ma_4pAdHubE

This patch undo's all closing brackets when typing what appears to be
general text. It does go further than just undoing " and ' chars, as
there are legitimate uses for singulars of the others too in general
text, such as a single ( where a user may want to type :-( and not end
up with :-()

Extra line here removes char to right (closing bracket) if user is
adding text under a 'both' case and if it has no token type (null etc)
at the cursor. Seems to be the cleanest and best way to do this. Undoing
what we've done isn't ideal, but we can only establish the token type
after adding our chars.

Should be a pretty inexpensive process tho and solves the issue well.