smartplaylist: allow exporting item attributes

[mgoltzsche]

Description

Allow generating m3u8 playlists so that they contain additional item attributes such as the id.

The feature is required by the webm3u plugin (M3U server) to transform playlists using a request based item URI template which may require additional fields such as the id, e.g. subsonic:track:$id.

To Do

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