Add Rasa NLU parser/matcher

[jacobtomlinson]

Description

This is my first attempt at #335.

This change adds a new parser/matcher pair for Rasa NLU. This is pretty standard and follows the same format as Dialogflow/LUIS/Wit.

However Rasa can also be trained directly through the API so this PR adds the ability to include intents in your skills in the Rasa markdown format (see the rasa branch of the hello skill for an example). Then when opsdroid starts it runs an extra train_parsers method which sends the intents to Rasa NLU to be trained into a model which will be used when parsing messages. This simplifies the portability of NLU skills as the intents don't have to be recreated and retrained by each user. This has also been added as a standard section of startup in the hope that something similar could be added for the other NLU engines. I have a feeling that LUIS can also be trained via API, not sure about the others.

Fixes #335

Status

UNDER DEVELOPMENT

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

• Run the Rasa NLU container docker run --rm -p 5000:5000 rasa/rasa_nlu:0.10.5-full.
• Configure the hello skill but using the rasa branch.
• Run opsdroid.

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #336 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #336    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files          17     18     +1     
  Lines         877   1023   +146     
======================================
+ Hits          877   1023   +146

Impacted Files	Coverage Δ
opsdroid/parsers/rasanlu.py	100% <100%> (ø)
opsdroid/core.py	100% <100%> (ø)	⬆️
opsdroid/loader.py	100% <100%> (ø)	⬆️
opsdroid/matchers.py	100% <100%> (ø)	⬆️
opsdroid/const.py	100% <100%> (ø)	⬆️

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