Adding stream metadata to `cmus-remote -Q`

Currently if you play a stream using cmus, you can see the
artist/album/title information in the curses display, but the same
information is not available by querying the server with cmus-remote -Q.

I wanted to be able to get to that information, and I think my way makes
sense ... basically I've added a new field (stream) to the output of
cmus-remote -Q which contains the stream metadata.

Here's an example of how this new functionality looks:

% cmus-remote -Q
status playing
file http://stream.mybox.com
duration -1
position 2
tag title Some Cool stream
tag genre Some Cool Genre
tag comment http://some-stream-url
stream Fila Brazillia - Brazilification Remixes 95-99 (CD2) - Freakpower / New Direction (Fila Brazillia Mix) # <- NEW!
set aaa_mode album
set continue true
set play_library false
set play_sorted true
set replaygain disabled
set replaygain_limit true
set replaygain_preamp 6.000000
set repeat true
set repeat_current false
set shuffle false
set softvol false
set vol_left 16
set vol_right 16

Incidentally, this field can also be used as a boolean to let a client
know whether or not a stream is being played.