Add filter_examples method

Created by: p-mongo

I am working on a parallel rspec test runner and it performs what effectively are multiple test runs in the same process. To achieve correct hook invocation I am using rspec's built in filtering functionality to set up filters to run a single example at a time.

The issue is that rspec loads examples and filters them in one operation, whereas I need to load them once and filter multiple times. The filter_examples extraction allows for this use case.