Option to not tune evals/samples

[awadell1]

Could benchmarkpkg have an option to not tune the parameters before test?

I tend to set them manuelly when I'm creating the benchmark ie.:

@benchmarkable foo(1) evals=300

Any suggestions on a workaround / could retune option be expanded to include retune=:never (or something)?

Thanks for the package!

Alex

PS: I'd be happy to put together the pull request for this change, but I wanted to touch base first