[#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
```
If we cleanly separate these parts of configuration, then we don't even need to reset any state, because runner can do that itself by default.

So, how about splitting `Configuration` into `Configuration` and `RuntimeConfiguration` (or better name, but I can't catch anything better right now). And then add ability to reset them separately. There are obvious configuration options, that should go to `RuntimeConfiguration` (like most of the things, that are set through command line and obvious internal parts, that are related to spec run itself) and options that should stay in `Configuration` (any default configuration, configuration set in `RSpec.configure` block, formatters).

In the case of such split, such things like inclusion and exclusion rules, formatters, that could be set when running specs, and that could be set in a spec helper (read: in a `configure` block) should be split into different options (one for configuration and other for runtime configuration), with runtime options taking precedence over former, instead of overriding and forfeiting them.

WDYT?