Add support for profiling benchmarks

[bheisler]

This looks like a useful project! I've had a few folks ask for profiling support to be added to Criterion.rs; now I can direct them to cargo-flamegraph.

That being the case, it seems like it would be useful to have the ability to run cargo bench inside a profiler with cargo-flamegraph. Is that supported? I didn't see it in the documentation. Naturally, it should be possible to pass arguments to the benchmark executables as well, probably using the same -- separator that Cargo itself uses.

[kstrafe]

This would be extremely useful