MPDUpdate and MPDStats could manage multiple servers

I'm using beets for a streaming server with multiple channels, i.e., multiple MPD daemons on different ports.
The solution, now, is to have many beets databases/configurations.

It would be fantastic to have multiple servers.

```
mpd:
server: myFirst
host: host1
port: 6600
password: mySecret1
server: mySecond
host: host2
port: 6601
password: mySecret2
```