Allow custom referer to work with Vimeo privacy settings

Vimeo Plus and PRO members have the option to add domain-level restrictions to their videos. Many of our customers at [Zaption](http://www.zaption.com) have done this with videos they uploaded to our web app. Now, those videos cannot be extracted by YTVimeoExtractor on mobile, because the referer doesn't match and therefore playback is restricted.

This update allows users of YTVimeoExtractor to add a custom referer to work with these restrictions.