Automatically install shell completions

[PeterJCLaw]

This repo provides shell completions for bash. It would be great if they could be automatically installed when the package is installed.
I had long thought this impossible, however I now think I've seen this done somewhere. I also recently discovered that it's possible to install man pages alongside a package (see e.g: https://github.com/srobo/tools or https://github.com/ipython/ipython), which suggests that something like this ought to be possible.

[PeterJCLaw]

I'm pretty sure pytest somehow does this.