Idea: Extensible neil add

[rads]

Right now the only way to extend the neil add subcommand is by making a pull request. In contrast, neil new can be extended with any deps.edn lib.

Would it make sense for neil add to work in a similar way where the user can provide their own “template” if needed?

My main motivation for this is allowing the community to extend neil without having to mess around with its internal code. If there is a particularly popular or useful neil add extension, that could be a candidate for being included as a built-in.

# Built-in command (unqualified name)
$ neil add shadow-cljs

# External command (qualified name)
$ neil add io.github.rads/shadow-cljs

[borkdude]

I like the idea!