Global cooldown for sound effects

[surlydev]

The stream could do with a global cooldown option on the sound effects.

!sfx off [optional time] [unit of time]
optional time could be 5 minutes
option unit of time, default would be minutes. Other values s seconds, m minutes, h hours
e.g.
!sfx off 2 h
!sfx off 30 s
!sfx off 5 m