Compare benchmark results with another framework in PRs

[Kaliumhexacyanoferrat]

The result table with the deltas is pretty cool. However, when developing a tuned version (e.g. to test parameters or a tuned new version), it would be awesome to be able to compare the current framework values with a another one.

For example /benchmark -f genhttp-11 --compare genhttp

In this case, the result table should compare the new values with the results of genhttp instead of genhttp-11 (or maybe even both).