Documentation refresh

Refresh of the documentation. See:
https://github.com/phase2/grunt-drupal-tasks/blob/docs/config/README.md
https://github.com/phase2/grunt-drupal-tasks/blob/docs/config/CONFIG.md
https://github.com/phase2/grunt-drupal-tasks/blob/docs/config/EXTEND.md

Resolves #11 and #16.