Factored extension points and handlers to be more generic. Buffering until waitForVendor is finished. Exposing api for setting user ids and properties at config time. Included setSuperProperties[Once] and mixpanel.register[_once]. Updated mixpanel plugin to order registries so that super/user properties and user id buffers are flushed before events. Defaulting the buffer flushing to run immediately so properties can be set before events are written.
Global setting for buffer flush delay, defaults to 0 so buffers are flushed before event tracking.