Asv compare variant

[Lothiraldan]

This is a Work In Progress command to be able to compare two variants on the same commit. We are starting to use it right now to validate the UX and use-cases.

I've tried to refactor the compare command to reduce the amount of duplicated code to a maximum.

Before putting more effort on the code, having your view if this feature is useful and what form should it takes would be invaluable. I'm hesitating between a new compare_variant command or a compare flag?

[pv]

I'd prefer retaining it all in asv compare command, e.g. syntax asv compare envname1:xxx envname2:yyy. The feature itself certainly should be added (closes #370).

EDIT: reading this again, it seems to be about parametrized benchmarks --- in which case --bench foo(param1, param2, ...) or similar would be the syntax, following other commands that support parametrized benchmarks. However, comparison across environments would also be useful.