Add silence_filter_announcements config option

Created by: draffensperger

We run specs with a couple of filters, but when I run them at the command like, I'd like to avoid cluttering the output with filter announcements like "Run options: include {:focus=>true}".

This add a new config option silence_filter_announcements (default false) which allows you to prevent the filter announcements from being printed when you don't want them to be.

This fixes this issue: https://github.com/rspec/rspec-core/issues/1896 and would resolve this Stack Overflow question: http://stackoverflow.com/questions/27698184/how-to-suppress-rspec-output-run-options-include-focus-true (as of now, 8 upvotes and 142 views)