Support setup/teardown functions

[seddonym]

I have some code I'd like to benchmark on codspeed which requires custom setup before each iteration. (If you're interested, it's testing the performance of a partially-populated cache which will get fully populated on each iteration, so I need to reset it to a partial state.)

pytest-benchmark supports this via a setup callback in its pedantic mode. It would be great if there was some way of achieving this using codspeed. (Might as well support teardown too.)