Automated "fallback 3rd party loader" in bash-completion upstream

[scop]

In bash-completion upstream, we've since version 2.12 started adding "fallback 3rd party loaders" for utilities that emit their own completions. This is so that users and app distributors don't really have to do anything extra to install completions, they will just work if bash-completion is installed.

I've posted the first implementation for a tool that uses this library for review a moment ago. It's for https://github.com/getsops/sops but that's not terribly important, other apps using this lib will make use of the same loader code once we get to add support for them.

Just posting here so that you are aware, and if you have some comments on the review: scop/bash-completion#1211