Implement my ThemeExtras plugin into the core

[johnstray]

Implement the functionality of my ThemeExtras plugin into the core, modernizing and improving Theme management.

Doing this will enable themes to provide custom fields, configuration settings and more, negating the need for each theme to carry it's own separate plugin, or the need to rely on multiple plugins (such as the original Custom Fields plugin).

This will allow for the development of much better, more functional and modern themes for the websites of today.