Reproducible randomness in setup for BenchmarkGroups #201

When working with random numbers or would be useful to have a way to guarantee that each benchmark in a group uses the same series of numbers. Unless I'm mistaken there is no way to do this now.

If BenchmarkGroup had a property setup::Function which was evaluated for each Benchmark, one could use setup=()->seed! (1234) to make sure that the benchmark runs use the same sets of numbers.

Or setupcould be an Expr so you could create a new rng.