[#1700] Introduce RSpec::clear_examples method

As a developer on such heavy platforms like jruby or similar
Since loading times of such platforms could be overwhelming
In order to be able to run my specs multiple times from the same process
And to be able to not bother reloading my non-standard tree of spec_*_helper files
I want to be able to clear defined examples without wiping my configuration


This one should fix this issue: https://github.com/rspec/rspec-core/issues/1700 
@myronmarston, comments are welcome!
@undefinedvalue, it would be great if you tried this out, feedback is welcome. You need to replace this https://github.com/hanachin/rspec-console/blob/support-rspec3/lib/rspec-console/runner.rb#L14-L15 with this:

```ruby
RSpec.clear_examples
```
I'm wondering if it might be better to model the configuration state differently to more cleanly separate that which applies to a single run vs that which should be carried across.