Add support for M3U playlists

[TacticalLaptopBag]

I have an iPod with Rockbox, and wanted an easy way to throw playlists from my desktop to it. Alternatives already handles the music, so why not the playlists? #116 shows others looking for this feature.

Currently, it's rather rudimentary. There's no smart merging of playlists, it just deletes all the playlists from the collection and recreates them brand new. However, it's good enough for some simple use cases.

This should adhere to the requirements discussed in #116, namely these:

1. For every playlist, there should be a corresponding playlist in the alternative collection
2. Every item in the original playlist has a corresponding item in the new playlist which points to the path in the alternative collection.
3. Users are able to configure the directory where playlists are written.
4. Users are able to configure which playlists are included on a per-collection basis. These can be specified as direct files, or directories containing playlist files.
5. Playlists in the alternative collection that are not specified in the config are removed.
6. Items in the playlist that are not in the alternative collection are removed when copying the playlist over.
7. Users are able to configure the file paths inside the playlist to be either relative or absolute

Potential Enhancements

• This could load playlists from the playlist and smartplaylist as well. These should probably be specified by the user which ones should be included. I wrote this before looking at the docs for these plugins 😅. The current implementation is sufficient for this. The playlist and smartplaylist plugins simply place M3U playlists into a directory specified by the user. The user could then just point the alternative collection's playlist to this directory, or playlists inside of it.
• Changes to playlists in alternative collections could be reflected to the original playlists. This could get dicey, as we have to account for items in the original playlist that aren't in the alternative playlist, and potentially have to deal with both files being changed in between updates. This could be a CLI option that simply overwrites the original playlists.
• Currently, no tests have been written for this functionality.

Closes #116