Added After event processed

[daehli]

Describe the event process, related to #25

@Gordon-BP

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[Gordon-BP]

What's a good use case for after_event_processed hooks? How are they different from after_incoming_middleware?

[daehli]

An event (Message/payload/content-type) can have multiple middleware(basic-skills/05_extract_slots.js,builtin/apply_nlu_contexts.js). Those middlewares can affect the event. The after_incoming_middleware will modify every middleware.

The after_event_processed is processed only one time at the end end of the event pipeline.

I don't have any good use case or any idea for the moment.