Feature request - support for remote imports

It would be great if Jsonnet could natively resolve remote imports, rather than be limited to `import`ing local files. In its current state, a user must use a workaround like defining all of their Jsonnet in a monorepo, or using a secondary tool like `jsonnet-bundler` or `vendir` to "vendor" the Jsonnet files locally first.

It would be great if we could extend [imports.go](https://github.com/google/go-jsonnet/blob/master/imports.go) to support remote imports. This could either be an extension of the default `FileImporter` and it's associated functions, or a new importer type. Extending the default importer would be preferred since it's [the default importer](https://github.com/google/go-jsonnet/blob/master/cmd/jsonnet/cmd.go#L454) in `cmd.go` today, otherwise users would have to change that one line and maintain their own fork/build.

I started working on a quick-and-dirty [implementation](https://github.com/google/go-jsonnet/pull/431) for resolving imports over HTTP, but this became very GitHub-specific once I added auth support. I'd be happy to develop this feature and submit a PR, but I'm hoping the maintainers could provide feedback on (a) whether you support/reject the concept of remote imports in general and (b) preferences on protocol (HTTP vs git vs other) before I invested more time in this.