Template registry viewing & selection

[srid]

Presently, om init lets the user select from a handful of templates:

This is not sufficient. If I want a simple Rust devShell, none of those templates are relevant. I could create a template repo for that Rust devShell, and then add it to the registry. But to help the user select between rust-nix-template and this one, the prompt should provide helpful descriptions to that end.

We need perhaps both description and tags in the template spec, and have them appear in this prompt.

An online registry (could just be simple static content in our mdbook) is also useful here.