add support for injecting polyfills

This adds support for injecting external JS polyfills on page initialization.  I need to do this specifically because [PhatomJS Date.parse is broken](https://github.com/ariya/phantomjs/issues/11151).  Instead of hard-coding polyfills, the user can just specify local paths to files they want injected, which will happen after [the page is initialized](http://phantomjs.org/api/webpage/handler/on-initialized.html)