Support for HTTP URLs as module sources

[radu-matei]

In writing the application manifests for fermyon.com, we have consistently seen the need to copy the built modules from Bartholomew or from the static file server, so they can be referenced in the manifest.

Ideally, we could reference a built module from a GitHub release, as opposed to having to copy the built artifact.

What should the configuration look like?

Original issue (cc @vdice in deislabs/wagi#135):

[radu-matei]

Closing in favor of #393, which is a bit more detailed.