Add callback functionality

[roytouw7]

Using Mockery there is the option to register a callback function which will get triggered upon hitting the mock, in our unit tests we use this to have the unit test wait for some mocks to get hit which happen in a detached go routine, this way we do not have to add any arbitrary time.Sleep in the tests. The Mockery example is listed below, it would be nice if this would also be possible in Gock.

test.reservationEmailService.
  On("SendConfirmationEmail", mock.Anything, &reservationModels.ConfirmationRequest{GUID: "123456", Language: ""}).
  Return(fmt.Errorf("email error")).
  Once().
  Run(callback)