Supporting successful grunt executions notification

Hi!

Needed an option to automatically support success notifications, e.g. when grunt target is completed without issues. Specifically wanted this for watch, so console can be in the background. My grunt setup is a bit contrived since it is a part of maven multimodule project and some tasks are generated dynamically, hence the need to notify of success by default.

Added the option to turn on this behavior, off by default. `lib/hooks/notify-fail.js` might be a bit wrong place for this (based on file name), but it felt most natural to place it there.

Also, added an option to set the duration option for notify-send.

Kudos for the plugin :-)