Add --excludes to run_benchmarks.rb to easily exclude benchmarks

[eregon]

--category and --name_filters don't give a way to exclude some benchmarks.
--name_filters seems already quite complicated so adding some negated filter there seems undesirable.
Therefore I think it's best to add an explicit --excludes=x,y,z option.

Thanks to the recent refactor, this is pretty easy :)

There are many possible reasons to exclude a given benchmark notably too unstable performance, known to fail on some Ruby version and no point to even bundle install, etc.
This is vastly more convenient than having to manually exclude by including every other benchmark by name, especially as more benchmarks are added.

[eightbitraptor]

I think this is a great idea.