Support for private/mentioned flag

Adds private flag for 1-1 messages.
Adds mention flag if contains user's nick.

Necessary for client to be able to filter on notification type, see https://github.com/Neamar/notiflow-client/pull/6
'notifyEventTypes' was just for use on L183, to prevent the notification from being sent on other events, so I didn't think of this.

I'll look into it tonight, thanks for the feedback!