Amplitude: added setDomain and setGlobalUserProperties to init

fixes https://github.com/segmentio/analytics.js-integrations/pull/448

@DirtyAnalytics split the setDomain and setGlobalUserProperties into separate methods to be able to test them. For each of these we have two tests, one that checks that setDomain (setGlobalUserProperties) was called, and another that checks that the setDomain method calls the window.amplitude.setDomain with the right params

@yields : @calvinfo and I wondered if there's a better way to test these two methods if we kept them inside initialize? 
maybe same thing here, otherwise lgtm!