Expose `urlsToWatch` as an option

[garrytrinder]

To enable the ability to support generic presets, like generating Open API specs, generating mocks, execution summary etc. we need to be able to provide URLs to watch at run-time.

Therefore we should add a new option

-u, --urls-to-watch

[waldekmastykarz]

When we implement this, we also need to update the rc schema to make the urlsToWatch property optional.