New flag for limiting format options to audio only

[fantesykikachu]

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It would be nice if there was a flag for limiting downloads to audio only formats, as opposed to using the -x flag which still downloads the video then extracts audio which can lead to a consumption of data. I am not a coder so I am not telling people how to do it; but there could be 4 ways of deciding on the flag, first we could give it an entirely new flag, or we could replace -x with this, we move --batch-file which currently uses the flag -a to like -b so the letters match, or in addition to making audio-only use -a we could improve detection of when a file is on the local machine and treat those as batch-files.

[dstftw]

-f bestaudio.