feat: add alternative source as optional argument

[12rambau]

As mentioned in another PR, sometimes the codec of the video is unsuported by the user browser. By adding an extra argument to the directive, one can add an extra video file (with another format). the first one compatible will be displayed.

• add a secondary argument option
• write the whole video tag on one line to improve readability (in python)
• add a configuration parameter to enforce alternative source (video_enforce_extra_source)
• add documentation of the behaviour
• add tests

CC @JulianGro for feedback

superceed #4

[JulianGro]

Yeah, this looks amazing. I am glad that there is a maintainer around for this that actually fixes stuff up. Once this is merged and I find some time, I can finally move out documentation to upstream video.