Add homer and antigen

Homer is a home directory manager that takes the same approach as Homebrew does for package management. It converts the entire home
directory into a Git repo, and provides a simple command-line interface for managing shell extensions such as plugins, `bin/` scripts and
aliases using this repository. It also provides a very light set of scripts that act as a framework for the user to build their own shell
environment.

Homer relies on ZSH and Antigen, hence why that formula has been added in this PR as well.
A versioned URL (e.g. pointing at a tag) is required.