Allow baseline scores to be provided in order to report performance changes

Performance reporting is usually performed within the context of making improvements to a site, therefore reporting changes from previous scores is important. This is why the myriad hosted services exist to report performance changes over time. There's also LighthouseCI to store and generate historical reports yourself.

These services can be overkill if you just want to see before and after results. It would be great if Bulk Lighthouse Tests allowed a baseline of scores for its URLs to be provided, and for its output to show the +/- change between them in its output.

I can see two potential ways to do this, both have their advantages and disadvantages:

1. Provide a "before" and "after" URL for each page, allowing both tests for a URL to be run at the same time
- This is beneficial if you have an existing production site online (eg. `example.com`) and want to compare it to an in-development site that's due to replace it (eg. `preprod.example.com`)
2. Provide existing scores for all URLs, which are used for the comparison
- This is beneficial if you have a set of results that you've already run with Bulk Lighthouse Tests and you want to run them again to determine the change