improve reports over time & by tag

[aretecode]

• compare date ranges (related https://github.com/fluents/bench-chain/issues)
• compare all averages with specific tags
• auto compare tags better - unique them fully and then group

[wmertens]

I wonder, would it be a good idea to use the current git commit as a tag? That way you know what code resulted in the test and you can group by commits, but OTOH you will end up with a ton of tags.