Add webhook parser

[jacobtomlinson]

It should be possible to trigger skills via a webhook. This could be implemented as a matcher and parser to allow users to directly decorate their skills to be called by a webhook.

To implement the RESTful API Flask could be incorporated into opsdroid and run at startup.

An example skill could look like

Skill

from opsdroid.matchers import match_webhook

@match_webhook("mywebhook", "POST")
async def webhook_skill(opsdroid, config, message):
  pass

Config

skills:
  - name: webhookskill

This skill could then be triggered with a POST request to http://host:port/skill/webhookskill/mywebhook.

Responding to the message could return the body to the webhook caller and the message passed into the skill could have a property of data which contains the POST or GET data if there is any.

[jacobtomlinson]

Closed by #96