[bigmarker] Add new extractor

[serl]

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BigMarker is a platform for webinars.

Unfortunately, I don't have many test URLs, that also means that I don't know if and how exactly extract more metadata.

[dstftw]

Read coding conventions.

[serl]

@dstftw I guess it was about author being explosive if missing?

Anyway, there I'm parsing some Javascript on the page, is there a better way or regex is ok?

Thanks!

[cypheron]

I guess because of the huge backlog @dstftw doesn't actually check any code, he writes that "read the conventions" nonsense over all PR's without ever mentioning a specific thing. Hope that @dstftw can get off his horse and does some work, otherwise I don't know if my excitement about ytdl being back was really appropriate.