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
this can be problematic in some edge cases, for instance let's say your name is Tom :expressionless: and someone write about the new "tomb raider": this will count as a mention :\