smartplaylist: change option --extm3u to --output

[mgoltzsche]

The boolean flags --extm3u and --no-extm3u are replaced with a string option --output=m3u|m3u8.
This reduces the amount of options and allows to evolve the CLI to support more playlist output formats in the future (e.g. JSON) without polluting the CLI at that point.

This is a follow-up of #5046.
Relates to #5048.

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.)

[mgoltzsche]

I renamed the option from --format to --output now to avoid confusion with the option with the same name within beets' ls command and to allow #5048 to define the --format option instead.