Add sidekiq_job_attempt_threshold config variable

The same idea as Tadas Tamosauskas's commit, but implemented differently and for Sidekiq. I am happy to make whatever changes you'd like.
I think this is a creative approach (and it has some benefits), but I'd prefer to keep as much 3rd party integration-specific code as possible in the *integrations/* namespace. Do you have any concerns about moving the threshold check into the Sidekiq error handler at https://github.com/honeybadger-io/honeybadger-ruby/blob/master/lib/honeybadger/integrations/sidekiq.rb#L33?