Adding cdnPath for own CDN-Handling

Hi,

i added an option "cdnPath" for own, single CDN-Handling.

In one project i run an own, cookie-less CDN to avoid useless data transfer
and speed up responses, but building the site with a grunt workflow locally..

With this changes i can tell cachebust to rate paths with this prefix as local.
After busting i can upload the assets to the cdn and the html-files to the webserver 
and everything works beautifully ;)

Maybe you want to implement it to your version.

Cheers,
lietzi