Feature Request: Alternate thumbnail embedder - tageditor

[DoctorVanGogh]

• I've verified and I assure that I'm running youtube-dl 2017.03.15

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What is the purpose of your issue?

• Feature request (request for a new functionality)

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Description of your issue, suggested solution and other information

I'd like to propose an alternative to atomicparsley for thumbnail embedding: tageditor (https://github.com/Martchus/tageditor). It's supports mp4 and mkv embedding of cover images (among quite a lot of other things).

Example command line:

tageditor -set cover="foo.png" -f "foo.mp4"  [--max-padding nnnn]

Might be able to resolve #10359.

If using an alternate external application for thumbnail embedding is already supported, at least --help doesn't mention it.

[Tatsh]

Looking into this because AtomicParsley as it is currently distributed is a bit dated. For those who want to build it locally they may have more luck but having an alternative is definitely good, especially if the only thing we use it for is thumbnails. MP4Box of GPAC could also be used, but that only supports MPEG-4 files.