CSPify stylesheets in the same as for javascript files

For use in chrome extensions, i needed to un-inline my styles into a stylesheet, just like for javascript files .. Came up with this proof of concept which works with Canary and latest versions of Polymer .. 

Opening a pull request for feedback, not sure if this is something that should be merged in the core , if so , maybe some better, more verbose configuration options would be necessary, since the csp: true option will have a different behaviour with style tags under this branch..