@dblock => Add optional retry_count argument

It would be nice to be able to specify the number of times to retry failing specs, esp. with intermittent capybara tests. I think it should be an optional argument to the 'rspec-rerun:spec' task which would default to 1 (current behavior). Let me know what you think- still working on a spec! (Trying to write a spec that checks the task that does a rerun gets called retry_count times when failing).