Add brew cask font support

I hope this is correct. In reference to https://github.com/kitchenplan/kitchenplan/issues/79.

You can add a font to your kitchenplan setup by using:

```yaml
attributes
  cask_fonts:
  - font-inconsolata
```

Check the [Cask fonts](https://github.com/caskroom/homebrew-fonts/tree/master/Casks) to see which fonts are available.