bench: allow users to run benchmarks non-interactively

[mmarchini]

This allows users to run benchmarks non-interactively by using command line flags. Useful when running benchmarks automatically on CI environments, for example.

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Kind of an alternative to https://github.com/fastify/benchmarks/pull/106/commits/9dc8d902ad44a3f37158f93cfb6ba230764c25da#diff-6f09bc424586c395b13fd6f33fc863bcR8-R69 using commander to handle CLI parameters. I only saw that PR after I made this commit, thought I'd share anyway in case you want to go with this approach instead.

Checklist

• [x] run npm run test and npm run benchmark
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message and code follows Code of conduct

[mcollina]

Would you mind to rebase/update this? I think #106 made some of these already.

[mmarchini]

Will do when I'm back from vacation