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.
Major changes:
- filter ``embed`` is replaced by tag ``video``.
- removed caching for whole backend, caching properties instead.
- HTTP or HTTPS is set by context.is_secure() (just for ``video`` tag).
- backends can be HTTP only (``allow_https = False``).
Minor changes:
- changed theme of docs to RTD theme.
- help about testing HTTPS on development server.
- added and modified tests
