Provide option to force src change in requirejs tag

Not sure whether this is the most semantic way, but I think there should be an option to ignore RequireJS format and replace src with the one provided in usemin comment block.

You can find here an example of usecase: https://github.com/sergeylukin/emmet.docpad/commit/9df7443b248c2c2716e1045214217bde8c3f551e

Hopefully this makes sense