Allow for the mute duration to be configurable

[PauloGDPeixoto]

A configuration to allow us to prolong the mute duration would be nice, since sometimes we are not typing that fast and it results in whoever's on the other side hearing the first keypress before mute multiple times.

[joaopslins]

I have a similar problem: The mic unmutes too fast and the last keystroke goes through the mic. I would like to configure the time after no keystrokes where the mic is muted.

[Noloxs]

I have made a pull-request that might solve your problem. https://github.com/stuartlangridge/hushboard/pull/26