Add -embed flag into mockgen

[stoikheia]

This proposal is to add an option -embed into mockgen.

Why: Sometimes we need to embed the source's interface (or structure) into a mock.

For example, a grpc server takes a mocked server implementation as an argument and determines if the interface is actually embedded as follows:

if !st.Implements(ht) {
    logger.Fatalf("grpc: Server.RegisterService found the handler of type %v that does not satisfy %v", st, ht)
}

https://github.com/grpc/grpc-go/blob/56ac86fa0f3940cb79946ce2c6e56f7ee7ecae84/server.go#L671

The following sample will result in an error when executed. https://github.com/stoikheia/GomockProposalSample1/blob/517c5bb860be39c9d2fa4faff6b0542fe29a7f3e/server/server_test.go#L26-L36

I think that -embed like option is necessary from above.

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