Allow templates to be downloaded from remote repositories

[MaxDesiatov]

This would allow users to create their own templates, but also would allow us to fix templates quickly without forcing our users to update carton on their side.

[MaxDesiatov]

I've created a basic template repository, which also has a textual introduction with links for beginners, and a working test suite. Still need a similar template for Tokamak.

[j-f1]

Perhaps this could be implemented like Homebrew taps? user/name would download https://github.com/user/carton-template-name/archive/main.zip, with user defaulting to swiftwasm if not provided. Alternately, users could pass a full URL to a zip or tgz file that would be unpacked.

[MaxDesiatov]

Yes, totally! 👍 I think we'd keep basic and tokamak as aliases for two corresponding repositories, but would be great to support what you've described as a general form. For whoever wants to implement this, the implementation obviously can span multiple separate PRs.