Fully document playlist format

[ssssam]

We should have human-readable documentation of the playlist format.

We should also have a way of automatically validating a playlist.

So far the format is as follows:

• 'track' entries are 'normal'
• 'album' or 'artist' entries can also exist, as a useful shorthand
• standard keys don't contain a '.'; tool-specific keys do

Some points to consider:

• using '.' in key names doesn't play well with jq
• at one point I thought it was a good idea to have multiple tracks inside a single toplevel item. cpe tracker local-albums still does this (#17 should fix that). Since we switched to the json-seq format, it's become obvious that nesting multiple tracks inside of one toplevel json object will negate the benefits of that format.

[ssssam]

I'm wondering about copying the properties from the XSPF playlist format. This should save some effort vs. defining our own spec.

There would need to be a few changes in property names e.g. artist -> creator.

It would be nice if we could use XSPF for everything instead of having a custom format. But this would mean that we couldn't stream partial playlists any more, which for me is a crucial feature.