Add a way for manually triggering Subsonic library update

[tuukkao]

Proposed solution

• Add a new command subsonicupdate (no arguments) which will trigger a library update for the configured Subsonic server.
• Add a new plugin config parameter auto (boolean, defaults to `false). When this option is enabled the update request won't be made automatically with library or smart playlist updates.

Objective

Goals

• To allow updating Subsonic library explicitly, for example if the library is on another machine and is independent of the main Beets library

Non-goals

Anti-goals

• Nothing should break since the new auto config entry would be false by default which maintains the existing behaviour.

[tuukkao]

My use case: I maintain a Subsonic (actually LMS which provides a (Open)Subsonic api) library on my server. I have only a subset of my music collection there which I transfer to this machine with the Beets-alternatives plugin. For this reason it's not useful for me to trigger the library update automatically with each Beets db update, since any changes to the Subsonic library happen manually. I'd like to have a way to disable the automatic updates and instead perform the update manually after syncing my Subsonic library.

I already have a working draft of this feature and would be happy to provide a PR if you think this is worthwhile.

[arsaboo]

I will try to add this to the core. In the meantime, feel free to check out beets-subsonic plugin.