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.
Please make sure this installs to `prefix`, not /usr/local.