Notification on mention

[akavlie]

Add support for user notification of any channel mentions

A couple of ideas for notification methods:

• Add a count of unread mentions (for all channels) in title
• Play a sound effect

[akavlie]

A bunch of JS audio solutions mentioned here:

http://happyworm.com/blog/2011/11/15/html5-audio-apis-how-low-can-we-go/