Add support to modify WP Customize default sections

I often find myself removing, modifying and adding to the default WP Customize options. I've written a bit of code to make turning off default sections easy, along with bits of boilerplate for modifying them. 

Still need to:
- Write js for the postmessage transport method
- Create some sanitize functions to sanitize inputs
- Create some boilerplate Sections, Controls and Settings

My first Pull Request, so be nice :)