ffmpeg: Add demuxer options.

[j0sh]

This adds demuxer options as a complement to the existing encoder/muxer options which allows us to

1. explicitly select the demuxer to use if probing doesn't return a good result, and
2. configure the demuxer with additional options

This has come up a few times while looking at various things so it is good to have an API that is fully configurable out of the box.

This was partially implemented in #420 but this is more complete - it actually allows for demuxer selection and adds test cases.

[leszko]

@j0sh Could you add a description to the PR to add the context ( 1) Why this change? 2) What is it changing from the "user" perspective? ). Thanks!

[j0sh]

@leszko

Updated the PR description. For more color as to "why" : I have been testing some flags to tune for latency but the gains are marginal so I am not sure we will keep them. eg, nobuffer allows us to drop up a few frames at the beginning to stay closer to the live edge, but this only really helps if you're playing back from the very beginning. if you start playback mid-stream then you're already starting at the live edge and it doesn't do anything.

BTW tests seem to be failing so I will hold off on merging this until I have some time to make CI green.

[j0sh]

Tests are passing locally; the CI failures are unrelated, see #421 (comment)