Add Homebrew formula

[apjanke]

Since ronn-ng is a binary that comes with shell completion files and the like, a straight gem install isn't adequate to get it all set up. Better to have it distributed by package managers.

Create a Homebrew formula in a custom tap to get it started.

Need to figure out what to do for Linux, too. I don't want to maintain packages for the various Linux distros.

[hosiet]

You may leave packaging issue in Linux distributions to distribution developers. @apjanke Please consider opening a separate issue and we may discuss in detail there.

[apjanke]

Cool. Here's an issue for discussing Linux packaging: #21.

[apjanke]

Homebrew formula created.

Custom tap here: https://github.com/apjanke/homebrew-ronn-ng.
Added installation instructions in 5cb61ca.