Allow running against a single dashboard

 - add a new command line option --dashboard that allows running against
   a single dashboard
 - uses a thunk that wraps the spotlight.dashboards function call
   passing in correct arguments.
Only download the dashboards we need

- If we're specifying just a single dashboard, then let's just get that dashboard from stagecraft
Merge pull request #95 from alphagov/feature/run-single-test

Enable testing against a single dashboard