Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
For these sorts of lazily initialized attributes the ivar is usually named the same as the method.  I think `verifying_double_blocks` is a better name then `on_declaring_verifying_double` because frankly it's not obvious from the name how `when_declaring_verifying_double` and `on_declaring_verifying_double` differ.

Actually, maybe `verifying_double_declaration_callbacks` is a better name for this?