Global toolchain repositories, or installing from a remote URL

[xizhao]

It would be nice to have a tc registry (in the beginning just associates git repos w/ languages) so you could do src tc install rust to get rust support.

[sqs]

Yes, FYI see #174 for something that will help here.

For now you can of course do src toolchain get example.com/repo and it will clone from https://example.com/repo.git.