Generic Notifiers

Having generic notifiers built into grunt would be awesome. Similar to guard (https://github.com/guard/guard/tree/master/lib/guard/notifiers), only stripped down to just an interface or base for core; then environment specific notifiers would be maintained as their own plugin.

There are already a few notification plugins for systems such as Growl or OSX system notifications, but those are very specific. Having a generic interface in core to provide a notifications system for tmux, emacs, dev tools, system events, irc, et cetera to plug in to would be very nice.