Add static interface implementation check

Add an additional line of code per interface to the generated mock file,
which allows the go compiler to statically check if the mock implements
the mocked interface.

example/mockpersonstore_test.go

@@ -13,6 +13,10 @@ var (
 	lockPersonStoreMockGet    sync.RWMutex
 )
 
+// Ensure, that PersonStoreMock does implement PersonStore.
+// If this is not the case, regenerate this file with moq.
+var _ PersonStore = &PersonStoreMock{}
+
 // PersonStoreMock is a mock implementation of PersonStore.
 //
 //     func TestSomethingThatUsesPersonStore(t *testing.T) {