[#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
```
So, the usecase is about using Runner multiple times with different set of arguments, so between runs, you want to reset what is included for run and what is excluded, because these rules will be set by Runner (appended, actually)

I need this when I want to run in my console such things:

```
RSpec::Core::Runner.run(["spec/model/something_spec.rb:7"])
```

and a few minutes after:

```
RSpec::Core::Runner.run(["spec/model/something_spec.rb:9"])
```

If I don't clear inclusion rules, it will include both lines, while I want to include only specified in last Runner.run call.

About they are the part of the config: what do you mean? When we use these: `Configuration#inclusion_filter=` and `Configuration#exclusion_filter`?

Then there is a problem: how to distinguish rules set by `Runner` and rules set by `Configuration`. If it was possible, then it will be perfect to drop only former.