Homebrew

Could be nice to have our releases available in [homebrew](https://brew.sh), so mac users can just `brew install gscloud` to install the client.

- [How-To-Open-a-Homebrew-Pull-Request](https://docs.brew.sh/How-To-Open-a-Homebrew-Pull-Request)
- [Formula-Cookbook](https://docs.brew.sh/Formula-Cookbook)
- [Acceptable-Formulae](https://docs.brew.sh/Acceptable-Formulae)

Considering hcloud (hetzners client), google and others have their clients in homebrew it shouldn't be a problem to get ours in there as well. As an example, [here's the hcloud formula](https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/hcloud.rb).