Support the --filter argument with VRT

[dalin-]

Typical scenario:

1. Run VRT
2. There's a problem in just one or two scenarios
3. Try to fix your code
4. Re-run VRT. You really only want to rerun the two scenarios that broke and not all 20. If you were calling backstop directly you'd do something like backstop test --filter="my broken scenario"

However

$ fire vrt:run --filter="my broken scenario"
The "--filter" option does not exist.