Add dialog dependency

@charlierudolph @allewun 

- reorganize and group formula settings
- automatically add "dialog" tool when installing Git Town (this can be omitted with the `--without-dialog` option)

More info about this here: https://github.com/Homebrew/homebrew/blob/master/share/doc/homebrew/Formula-Cookbook.md#specifying-other-formulae-as-dependencies