Added `video_embed` tag that accepts 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).  Created a tag that
takes keyword arguments and embeds them into the URL query string.

This tag also supports appending to existing queries hard-coded in the
video backend URL (e.g. Youtube's backend has a hard-coded query of
wmod=opaque).