Add originalyear support to MPC status

[chowbok]

Now that MPD supports the "originaldate" field, can we have MPC support it so we can do mpc -f "%originaldate%"?

[MaxKellermann]

All new core tags are automatically supported by mpc (and ncmpc); all you need is a libmpdclient version which supports the tag. libmpdclient 2.12 will support it, which is not yet released - but you can use git master until that happens.