Add config for Hydra (OSX window manager)

Hydra is a window manager for OSX. There are other window managers out,
but Hydra fits my needs pretty well:

- it's highly customizable
- configs are written in Lua (which makes configs easy to read and
  write)
- it's lightweight (always nice)
- it's under active development (unlike Slate which was last updated in
  2013)

Github repo: https://github.com/sdegutis/hydra