A way to enable/disable plugin

This pull request is for issue #11

Adds a new config variable ```enabled``` and defaults it to ```false```
Adds a warning message on install if the plugin is disabled and a Berksfile exists

@le0pard - this overlaps with your change from issue #9. I cherry-picked part of your commit and updated it based on the discussion in issue #11

I will add a pull request to Berkshelf to compliment this change when the Vagrant file is generated.