Add filesink to GStreamer output options.

[tonurics]

Please add filesink as a output option under GStreamer.

That will allow support for sending Strawberry's audio to a snapcast server or netcat as a pipe to a remote system. An example of usage can be seen here [mopidy also uses GStreamer].

Alternatively, a dedicated snapcast option would equally be nice; as only the filesink location would need to change.