Proposal to resolve Issue #116 without generating ISGOMOCK Method in structs

*Requested feature*
I want to resolve https://github.com/uber-go/mock/issues/116 without generating ISGOMOCK method.
ISGOMOCK method was implemented by https://github.com/uber-go/mock/pull/144.

*Why the feature is needed*
I think it is disturbing that the ISGOMOCK method for testing is generated in every struct.

*(Optional) Proposed solution*
Do you have any good ideas?