Add Feature: Local Customizations Folder (to simplify maintenance)

**Problem**

Zsh configuration is very personal. The way "oh-my-zsh" deals with customizations has created an explosion of pull requests some of which are probably too narrow to be in the main repo. This is a maintenance burden. Imo, only generic functionality should be added to this repo - not personal adjustments and tweaks.

**One solution**

Allow users to make tweaks outside of the repo.  This will deincentivize fork/pulls for personal changes.

In this pull I added the concept of 'local customizations'. Instead of adding tweaks to `~/.oh-my-zsh` you can instead add them to `~/.zsh`. You can then separately track this folder in a personal repo. I added mine to my dotfiles repo, for example.

This is backward compatible with the `/custom` folder, but imo we should phase that idea out. Also, adjustments in the `~/.zsh` folder have precedence over those in `/custom`.