[Shock][CLI] - Make root URL dynamic

[matthew-carroll]

As of the work for #180, a rootUrl can be set within StaticShock or at the top level of _data.yaml.

This value is set explicitly by the developer, so it's static. The problem is that when running locally, the root URL is localhost. As a result, anything that truly cares about the rootUrl will point to the wrong place, when running locally.

Revisit the rootUrl concept and rework it so that local running works, too.

One possibility is to receive the rootUrl from the shock build, e.g., shock build --root-url=localhost:4000.