Add automatic Closebrackets function to Codemirror.

This is a trivial change to add automatic close of brackets `{ `, `[` and `(` to code cells...
This behaviour is extensively spread across multiple IDE and code editors, and I think it worth to make it a default option.
I tested in live and I really like the millisecond I do not spent looking for the closing bracket :wink: 

Thoughts?