Chromium Web Browser Support

I'm new to git/github, so hope what I'm doing is right...

This adds detection for the Chromium web browser, which only contains open-source video codecs (i.e. WebM and Theora). Unlike Chrome, it has no native support for MP4 files.

This is also related to a 3 year old (unresolved) issue, "Chrome vs Chromium video source confusion" <https://github.com/johndyer/mediaelement/issues/62>.

Feel free to improve the code, as I'm not much of the "programmer type".