Show summary / comparison

[sharkdp]

If there are multiple benchmarks, show a short summary of the results like command1 is a factor of 1.5 times faster than command2.

[sharkdp]

We could also show a "highscore"-like list or table with the fastest command at the top. In addition to the absolute times we could show relative factors here.

[vks]

It would also be nice to do a statistical test whether the difference in runtime is significant. I believe criterion.rs supports this.

[stevepentland]

Now that we are collecting the times for all benchmarks together for export, we have a place to calculate this comparison built-in. This may be a good chance to consider implementing the functionality.

[sharkdp]

Closed via #50. I will open a new ticket to track the statistical significance test.