Py2/py3 script that can download macOS components direct from Apple

Thanks to:

* FoxletFox for [FetchMacOS](http://www.insanelymac.com/forum/topic/326366-fetchmacos-a-tool-to-download-macos-on-non-mac-platforms/) and outlining the URL setup
* munki for his [macadmin-scripts](https://github.com/munki/macadmin-scripts)
* timsutton for [brigadier](https://github.com/timsutton/brigadier)