smartplaylist: add extm3u/extinf/m3u8 support

[mgoltzsche]

Description

Adds the boolean configuration option smartplaylist.extm3u to allow generating extm3u playlists that contain additional information (artist, title, length) per track. See Extended M3U (Wikipedia) and playlist specification for HTTP Live Streaming (RFC8216).
This is to be able to display meaningful metadata and search tracks within a playlist within a player without having to load the linked audio files of that playlist.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[Serene-Arc]

Thanks! This is a very useful change, and it all looks good to me.

[mgoltzsche]

I realize it is disadvantageous to chose a boolean flag to specify the format because we may as well want to specify JSON as playlist output format at some point.
Thus, do you mind if I create another PR to change the newly added flag (before it is released) to --format (or so) and make it of type=string to let users specify --format=m3u|m3u8?

[Serene-Arc]

Sure! Go right ahead.

[mgoltzsche]

Okay, I created #5053 now to change the option.