Ability to set a "reference" job

Right now when benchmarks are compared, they are always compared to the fastest run. I think it would be useful in some situations to set a "reference" run and compare all the other runs to that.

One example would be the benchmark suite of `jason`, where I'd like to see all the other encoders reported relative to Jason itself, even though `jiffy` is the fastest most of the time. Right now, I have to do the additional computations manually, to get the desired numbers, that I can use to annoy people convincing them how fast Jason is 😉.

No file or test is named. Trace the benchmark comparison path that selects the fastest run, then inspect how runs and encoders are represented; done means a chosen reference run can be used for all comparison results, including the Jason benchmark example.

Written by the indexing model from the issue text.