Add support for insertPragma, like jsx-loader for webpack

This should help with pull request #14 (Added 'addDocblock' option which
will add the /** @jsx React.DOM */ comment to the source when it's
missing like the 'reactify' module does) but has the added benefit of
matching webpack's option style, and lets you choose whether or not you
want React.DOM added or not.  Also seems like a simpler pull request
(albeit less correct?)