Add remote control for Glados volume

[marksteward]

The required volume varies a lot between a Tuesday evening and a quiet afternoon. It can get annoying quickly.

[zymurgic]

An interesting suggestion. It's also been suggested that doorbell sounds should have a separate software volume control. An irc interface on robonaut might work, with mqtt communications that audio announcer listeners can respond to appropriately. Default to revert to 'normal' volume after a period of time.

Something like
?gladosvolume 50%
?gladosvolume 40% 2:30 to set it to 40% for 2 hours and 30 minutes - suitable for radio exams etc.
?doorbellvolume 80%