Added --local flag to use a local Cabin theme

The purpose of this option is to allow theme authors to test their themes during development by pointing Cabin at their local theme folder and to allow for faster/better integration testing. Previously the integration tests were pulling down the Candy theme which is constantly evolving, so it is much more consistent and manageable to have local themes to test against inside the Cabin repo.

In essence, the code change supports an additional `local` parameter which changes the behaviour of reading the cabin.json file and copying the theme. I also added Candy as a theme fixture, and updated the integration tests to only do one remote theme fetch and run the rest locally. Huge speed gains were had!