Add beforeClone

beforeClone gives the user a chance to make changes before @extractClone
is called. This can be helpful to unbind and clean up the DOM before
performing the clone.

Note: new .js compiled with coffee 1.6.1 (I'd be happy to recompile w/ 1.4.0
if you don't want to include the version jump in this PR)

ps Thanks for this great JS! It's been very helpful.