Add output of time spent on completing the task.

[inakrin]

Could you please add output of time spent for completing a task?
In grunt-watch there is output like this:
Completed in 39.909s at Sat Sep 23 2017 08:54:57 GMT+0000 (UTC) - Waiting...
While in grunt-simple-watch it's just Running "simple-watch" task
Simple Watch, Waiting...