Enabled support for default white background as a configuration paramete...

I'm using webshot to manage on the spot and queued screen capture jobs of user provided websites.
Playing with some big bulk capture I've realized (late) some of the websites were relaying on browser defaults for background, which PhantomJS and the jpeg output didn't like much (default to black). 
I've quickly integrated the option in your library, you may want to consider the support of it since I've realized I wasn't the only one bumping in this exception.
Style element is injected before anything else, so that any CSS provided indication will override the default to white and it can be easily extended to support a user provided fallback background color instead a boolean for the white one...
Cheers