Add an option to save a benchmark run for later comparisons

[CosmicHorrorDev]

There are times when I want to have some baseline saved that I compare later changes to. It would be convenient if I had a way to save a benchmarking run and then load that to later runs. Something like

$ hyperfine --save-benchmark baseline 'sleep 1'
$ hyperfine --load-benchmark baseline 'sleep 0.5'
{{ display things as though `hyperfine 'sleep 1' 'sleep 0.5'` had been run }}

If you're interested in supporting this then I can work on adding it as a feature

[sharkdp]

That sounds like a cool idea, thank you. It also sounds like a rather complex feature that is not easy to design. So before we go ahead trying to implement this, I'd like to discuss how this would work, what kind of implications this has on other features, what the CLI would look like, etc.

Note: we already have --export-json, so that could probably be used for the "storing" side of things.

[sharkdp]

Slightly related: #577