Add option for custom metadata

[srussel]

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Description of your issue, suggested solution and other information

I would like to be able to add additional metadata to the video file. For example currently I can do:

youtube-dl --add-metadata --merge-output-format mkv https://www.youtube.com/watch?v=iqKdEhx-dD4

to capture the metadata. I would also like to be able to capture some of the extra fields that can be used in the filename template such as series/season/episode.

An example of what the option could look like is:

youtube-dl --add-metadata --custom-metadata "SERIES=%(series)" --custom-metadata "SEASON=%(season)" --custom-metadata "EPISODE=%(episode)" --merge-output-format mkv https://www.youtube.com/watch?v=iqKdEhx-dD4

and then in the postprocessor include the custom metadata with the -metadata option.