Fix docs: versionadded
Updated video template tag to accept kwargs

Some video backends support optional arguments embedded in the URL query
string that adjust how the video is displayed (e.g. Youtube supports
rel=0 which means don't show related videos).  Updated the template tag
to accept optional KEY=VALUE pairs that will be added onto the embedded
URL as a query string.  Also added support for specifying default query
strings on a per-backend basis (e.g. so that all YouTube urls have
?rel=0 added to the end).

Added appropriate tests and documentation.
Merge pull request #20 from bburan/bburan/embed-with-kwargs

Added `video_embed` tag that accepts kwargs
Fixes #24.