Fix “config compare” by fixing Hanzo.run

`Hanzo.run` was recently modified to ditch its return value — but we need it in the `config compare` command.

This pull request adds a `fetch_output` argument that makes the method use the *`* method instead of `Kernel#system` (because we must keep using `Kernel#system` to stream the command output).

We might want to refactor `Hanzo.run` later to use `IO.popen`.