Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
This class is still going to suffer from mutation problems.  It's just that it's mutating a local variable in this context's scope rather than a global constant (which is obviously better, but still has the underlying problem).

Instead, I'd recommend doing:

``` ruby
RSpec.describe do
  class ClassThatDynamicallyDefinesMethodsViaATriggerMethod
    def self.define_attribute_methods!
      def some_method_defined_dynamically; true; end
    end
  end

  it 'allows constants to be customised' do
    test_class = Class.new(ClassThatDynamicallyDefinesMethodsViaATriggerMethod)
    # the rest of your spec
  end
end
```

By using a one-off throw away subclass you avoid the mutation problem, as there's no way for anything to use the mutated subclass outside the one spec.