github gifv support

This extension adds support for gifv files in your github comments.
To use it simply add a link to a gifv file to your comment and it will be rendered by anyone who has the extension installed.
I wrote this because our team likes to post funny gifs on github - and github not supporting gifv files was really hurting productivity. Hopefully now we can get back to work.

At the moment github does not support gifv embeds in your comments (see https://github.com/github/markup/issues/458), which is why this extension was born.