Request for comments: Support for shell autocompletion

I would like to get your feedback whether you would accept a PR for shell autocompletion.

I am in the process of migrating a RubyGems from [gli](http://davetron5000.github.io/gli/) ([GitHub](https://github.com/davetron5000/gli)) to dry-cli. gli generates output for bash and zsh autocompletion e.g. `rubygem-command help -c` (see [documentation](https://github.com/davetron5000/gli/wiki/ShellCompletion)).

I am pretty sure `hanami-cli` would profit from such a feature as well.

Should I take a shot at this or is this something you are not interested?