Casking script [WIP]

A partial font casking utility which automates a few tasks. Given an archive, produces three stanzas:
- `sha`
- `version`
- `font`, for all included OTF/TTF files.

Particularly handy for large families (e.g. `font-input`, which includes 168 fontfiles).

As declared in the heading comment, we should likely expand this into a complete cask generator which may be included in a `brew cask create --font` command.

Review apppreciated, as `font_casker` is effectively a bash script ported to a Ruby method soup.