Add ability to search for specs at a URL location

[chris-sanders]

With #650 implemented a URL spec entry point needs to implement that supports mergable specs. This may replace the current URL entry point or could use a different pattern depending on the final implementation.

The desired scenario is to allow for collecting specs from a git URL and walking the remote structure to build the final spec.

The initial use case is to allow for a GitHub repository with folders for various modular specs. The user can then point to an entry point folder to include all specs from that folder down in the tree. The spec collection should follow the same flags and operate the same as the filesystem collector in #653, including allowing the user to specify more than one URL to collect from.