Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
It seems weird to put the rspec config in `lib/user.rb`..it would never normally go there.  Can it go in file under `spec`?

Also, is it worth showing the config option as `RSpec::Mocks.configuration.when_declaring_verifying_double` to show how it can be done with rspec-mocks on its own?  I can see an argument either way for that...