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.