Add option to exclude a runner

[julianrubisch]

This requires some explanation. IIUC you can specify which runners to run with -r, but pronto will actually use this to require a certain runner.

I wrote a ruby-standardrb runner https://github.com/julianrubisch/pronto-standardrb (see respective PR #364) basically only by subclassing pronto-rubocop (because that's what standard is, an opinionated wrapper around rubocop).

Now of course in that implementation I need to require the rubocop runner: https://github.com/julianrubisch/pronto-standardrb/blob/c5669db79609f4fba40ba04362fe37130ea0f930/lib/pronto/standardrb.rb#L3

which will also load (and thus, run) it and lead to conflicting outputs since the base rubocop configuration is quite different from standard.

so I could basically copy-paste everything from pronto-rubocop and not require it at all, but being all tuned to DRY implementations this really makes me want to tear my hair out...

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TL;DR: Runners cannot be easily subclassed.

[ashkulz]

@julianrubisch can you check if the linked PR (which is now merged) solves your problem?

[julianrubisch]

does look good!