way to escalate failure when comparing outside of pytest

[nico-ssi]

compare outside of pytest allows to compare 2 previous benchmark runs.

However, there is no way to do --benchmark-compare-fail outside of pytest.

Please consider providing a flag i.e.

pytest-benchmark compare 0001 0002 --fail=<stat>:<num>

Then return a non-zero exit status when the fail criteria is met.