Support for deleting specific stub

[jbgosselin]

Hi there ! Nice project !

I would like to use it in order to mock GRPC APIs during testing.
I want to use a mix of static stubs and per test stub that would be cleared at the end of the test.
The problem is that it is only possible to delete all stubs at once, including static ones.

I was thinking of adding an auto generated ID to each stub and being able to delete a single stub if necessary.

Might make a PR implementing that.

I just want to have your thoughts about it.

[jbgosselin]

Thanks a lot for the improvement on v2 !