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.
@joshuap thanks for getting back to me. I went ahead and worked up something in the sidekiq integration bit. The implementation came out uglier than the first go-around and I could not figure out how to easily add tests without significantly modifying the existing sidekiq integration test setup. Have not come back to it -- would like to see if you have thoughts about this implementation. 
https://github.com/cdale77/honeybadger-ruby/commit/806121e7d05942baecf558322595c9c8128bf79c#diff-fe76b21261958d1b7301b4f2f90adda7L35