Add support for average time on tasks

Feature enhancement
 
* Add support for showing average given the options ```--average``` or ```-a```
* Added screenshot showing the result when requesting the output of the average time used.
* Updated readme to reflect the new feature added.

_Line 26 to 30 in ```time-grunt.js``` got updated as a result of the editorconfig file. The editor replaced the spaces with a tab as the rules states._