[#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
```
Yes, it seems not very relevant here.

Actually this is done to include a `RSpec.configure` block before first `Runner.run`. It is not very relevant here, but it is relevant for typical usage of this feature (running `Runner.run` multiple times from script or console).

If you don't configure it before and you have some formatter configuration there, you'll get default (`Progress`) formatter first and on top of it, you get your configured formatter afterwards, so the output is doubled.