Make themeable function for _cdn2_get_flash_js

It would be super-nifty if, instead of a static function to create the javascript for flash video, there were a theme function that could be overridden easily.

Flow Player has all sorts of features we'd like to use, but not in every instance, so it would be lovely if there were a themeable function for building the js to call up the player - I'd suggest making this in addition to the static one, so that legacy code wouldn't break.