Restructure functions file

There's a lot of Genesis developers who follow the approach in the PR - hooking all of the setup into a `*_setup()` function attached to the `genesis_setup` hook at priority 15. It really works well as Genesis has no problems with this being done.

The function permits commented-out lines of code (I've left one in as an example), which can indicate where custom-header, editor-styles, image sizes, sidebar registrations, and other such non-function setup can also be defined, along with actions and filters.