Add support for alternate backspace keybinding.

Currently, when you backspace over a left or right paren (or quote or square bracket, etc.) using the <C-h> keybinding, parentheses balance is not maintained. The <C-h> backspace keybinding should behave the same as the <BS> binding.
Merge pull request #18 from bclube/patch-1

Add support for alternate backspace keybinding.