Bespoke templates

Usually, we are content to start a session from a standard, empty template, e.g., a Markdown template or a Racket template. But sometimes there might be an existing session that we would like to start from, while treating the new session as entirely distinct. For instance, the original session may be a demo of a certain software library, while the new desired session might be for experimenting with and learning more about that library, and we would like a convenient and familiar starting point (e.g., the existing demo code).

One option is to provide a fallback option, "Custom," in the `mindstream-new` completion menu, which delegates to another completion menu for selecting a folder from the filesystem. Then, that folder is simply copied, just like any other template, but, additionally, excluding any `.git/` subfolder that might be present.

Note that the proposed new design in #24 would allow us to start new branches on existing repos, but that would be for the purpose of iterating on and experimenting with variations of the existing session, whereas the present need is more about starting a completely new session from what happens to be the same starting point as the current state of an existing session. In particular, the present option would allow us to start from any existing folder, not necessarily mindstream sessions, making it a good way to "import" existing projects as mindstream sessions.