WIP: Loading external task options

@mikem and @contolini: Opening this PR to show you the proof of concept. I think we should wait to merge until we set up the rest of the common tasks, as well. I'll walk through it with you guys whenever it's convenient.

- Adds cf-grunt-config dependency to `package.json`
- Sets up `Gruntfile` to load task options from said cf-grunt-config
  package.

Dependent on @cfpb/cf-grunt-config#2 to work correctly.