Audio notification when contacts come online

Particularly, the rustling sound from Ricochet: https://raw.githubusercontent.com/ricochet-im/ricochet/master/sounds/online.wav

It's sooooo nice.

changed title from Featuer request: Audio notification when contacts come online. to Feature request: Audio notification when contacts come online.

added T: feature request label

changed title from Feature request: Audio notification when contacts come online. to Audio notification when contacts come online

marked this issue as related to #30