Adding new feature

Now it's possible to add a description to an alias. That will be passed to Grunt.

For example:

````
default:
  - 'test'
anotherTask:
  description: 'This is an awesome task'
  tasks:
    - 'foo'
    - 'bar'
````

Will then be passed to grunt so running `grunt --help` will show the description.

Fixes #39
Merge pull request #66 from Belelros/feature/39-descriptions-aliases

ability to add description to alias