mpdupdate: Use $MPD_HOST environment variable.

[tarruda]

This variable is not standardized, but is already used by the mpc which is the official command-line client. It would be nice if beets read it too, to avoid duplicate configuration for mpc users.

[nathdwek]

Much appreciated!
Could you please amend your commit in order to:

• Link to somewhere supporting the usage of this variable in the commit message (for example http://linux.die.net/man/1/mpc).
• Similarly to how mpc explains it, explicitly mention in the doc the precedence of the variable.

edit: Sorry for the misclick 🐭

[tarruda]

Done

[nathdwek]

Thank you very much!