[FR] Let SetUp and TearDown be std::function

[balshetzer]

Is your feature request related to a problem? Please describe.
My benchmarks are parameterized (not by numbers) and I register them dynamically using RegisterBenchmark for various combinations of parameters. Setup and Teardown need those parameters. Custom setup and teardown take function pointers so I can't pass functors or lambdas with the parameters curried in.

Describe the solution you'd like
Setup and teardown can be std::function

Describe alternatives you've considered
manually creating all my benchmarks, setup, and teardown functions for each set of parameters. but that is not sustainable