Selecting audio formats as well as other improvements

[christoph-heinrich]

I've used this great script for over a year now and occasionally made some changes.

Recently I added the ability to change the audio format independent of the video format and decided to upstream all my changes. I tried to break up my changes into several commits for easier review, but adding audio format selection was so involved that there is one larger commit in there.

It also contains the changes of one other pull request. https://github.com/jgreco/mpv-youtube-quality/pull/12

Besides audio format selection, it now displays the bit rate and file size of each available format. It now also shows the automatically selected format as selected when opening a video, even if your default format string is very complicated. The file path gets checked, end if it isn't an URL, nothing happens. If downloading available formats fails the first time around for whatever reason, it tries again the next time the menu gets opened (this would have already worked upstream, except there was a bug in the error handling). It can also reset the format string to the original one when changing files. If the file has been opened before (e.g. going back in a playlist), it will set the previously selected format instead. Formats are downloaded as soon as the file is opened, so one doesn't have to wait for the menu to appear.