"--sub-lang" option should accept "/" as "OR" operator

[35609902357]

In the configuration file user-provided subs can be preferred to auto-generated subs by passing both options in the preferred order

--write-sub --write-auto-sub

but it doesn't work for --sub-lang en --sub-lang de.

Often user-provided subs are not in the same language of the auto-generated subs (for example user-provided are en-GB, while auto-generated are en. This leads to skip the user-provided subs and download the auto-generated ones. Passing en-GB,en would embed both tracks, which is an undesirable outcome, especially since they are both listed as "English" in the output file.

Adding the / to give an order and download the first available only (as it is for video formats) would allow to have a configuration file like

--embed-subs --write-sub --write-auto-sub --sub-lang en-GB/en-US/en

or

--embed-subs --write-sub --sub-lang en/de/fr

and not to worry checking available languages every time.