Add a way for manually triggering Subsonic library update

### 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.