Add the ability to run a workflow from an https url

[lodejard]

Simplest case is a path to folder of static file server, like: https://raw.githubusercontent.com/Microsoft/Atlas/master/examples/202-throwing-errors

Should recognize related folder https://github.com/Microsoft/Atlas/tree/master/examples/202-throwing-errors and change it to the related form automatically.

Maybe should also recognize if the path ends in /workflow.yaml and remove for convenience.