Intents/Slots history for awareness

[YuukanOO]

We need to provide an history of intents with their slots in the Agent.

With that in mind, when parsing a new sentence, if some slots couldn't have been extracted, we could try to look for them in the recent history before calling the skill handler.

The idea behind this is to be able to make something like this:

- Turn the living room TV on please
- Raise the volume <--- No identifier specified but since we previously mentionned "living room TV", use that instead
- Turn it off <--- Same as above

As a side effect, refering to #4 :

- Lower the temperature in the living room and turn lights off <--- Refers to the "living room"

should work too :)

Open questions are:

• Should it always try to fill needed slots? Only if created in a predefined interval? Only when multiple intents in the same sentence ?
• What are the bounds of the history ? Keep it forever? last 5 intents?

Regarding the implementation, nothing breaking here, a method such as get_available_slots(intent_name) should be added to Interpreter and returns the list of possible slot names given an intent (as per the training data)`.