Add an alternative formatter.

Formatting durations as 'XX:YY' is fairly common and repeated
in nearly all the nowplaying sources. Make a util method instead.

Change-Id: I8663a0ff5da7a17d7d47cf10e5b0e74722671b46
Reviewed-on: http://gerrit.dmdirc.com/3986
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>