autopair

Another stab at making braces and quotes pair like in
TextMate:

* Opening braces/quotes are autopaired;
* Closing braces/quotes are autoskipped;
* Backspacing an opening brace/quote autodeletes its adjacent pair.
* Newline between newly-opened brace pairs open an extra indented line.

Autopair deduces from the current syntax table which characters to
pair, skip or delete.