Can clap be made optional?

[faern]

Is clap really a hard dependency for criterion? I personally only use it as a library and don't specify my benchmarks over the CLI at all. clap pulls in a decent amount of dependencies that I don't think I'm using at all.

[stefnotch]

I think this would nicely tie into https://github.com/bheisler/criterion.rs/issues/426 as well. Currently criterion ships with more dependencies than necessary.