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
idea: instead of having notifyEventTypes as an array, why not use an object with custom handling?

```js
notifyEventTypes = {
  'message': function() {},
  'file': function(message, data) {
    var content = message.content && message.content.file_name;
    data.content = "[file: " + (content || "attachment") + "]";
    data.special = true;
  }
}
```