Update notify to extend options with optionsOverride first.

Hey guys, 

This change will allow for preventDuplicates to be recognized as an override option.  Use case for this is that I have a function that gets called when a user takes an action, which then calls to toastr.  This specific action is one that I'd like to use preventDuplicates on, without setting it to be true in toastr.options.  Currently, preventDuplicates is checked before extending options with optionsOverride, so my preventDuplicate override will be ignored when used in this manner.