Support specifying interfaces to mock when using source mode

[dhrubabasu]

Requested feature A clear description of the desired feature and an example of
how it would be used.

As far as I can tell, there is no way to specify which interfaces to mock out of a file when using source mode

Why the feature is needed A clear description of how this feature is not
served by existing functionality in gomock.

The closest you can get to this functionality is manually passing in every interface other than the one you want to mock in -exclude_interfaces

(Optional) Proposed solution A clear description of a proposed method for
adding this feature to gomock.

Adding an -interfaces flag which auto-computes the -exclude_interfaces flag would be relatively straightforward I think.