WIP: Git based app blueprints

Just a RFC for the general idea.

Needs tests.
`quick-temp` puts them in `./tmp` but I don't want to pollute user space with temp folders for app generation. `temp` uses whatever the system identifies as a system-wide temp location and cleans up after itself. `quick-temp` makes sense for generation once the app exists, but I think it's icky before the app exists.