Add Homer

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.