triggers implementation (#62)

docs/triggers.rst

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+Triggers
+========
+
+The package provides ability to write custom extensions which will be run on (the most) actions, e.g. after updates. By default ahriman provides two types of extensions - reporting and files uploading. Each extension must derive from the ``Trigger`` class and implement ``run`` method
+
+Trigger example
+---------------
+
+Lets consider example of reporting trigger (e.g. `slack <https://slack.com/>`_, which provides easy HTTP API for triggers for integrations).
+
+In order to post message to slack we will need a specific trigger url (something like ``https://hooks.slack.com/services/company_id/trigger_id``), channel (e.g. ``#archrepo``) and username (``repo-bot``). It can be retrieved from the same application instance.
+
+As it has been mentioned, our trigger must derive from specific class
+
+.. code-block:: python
+
+   from ahriman.core.triggers import Trigger
+
+   class SlackReporter(Trigger):
+
+       def __init__(self, architecture, configuration):
+           Trigger.__init__(self, architecture, configuration)
+           self.slack_url = configuration.get("slack", "url")
+           self.channel = configuration.get("slack", "channel")
+           self.username = configuration.get("slack", "username")
+
+By now we have class with all required variables.
+
+Lets implement run method. Slack API requires positing data with specific payload by HTTP, thus
+
+.. code-block:: python
+
+   import json
+   import requests
+
+   def notify(result, slack_url, channel, username):
+       text = f"""Build has been completed with packages: {", ".join([package.name for package in result.success])}"""
+       payload = {"channel": channel, "username": username, "text": text}
+       response = requests.post(slack_url, data={"payload": json.dumps(payload)})
+       response.raise_for_status()
+
+Obviously you can implement the specified method in class, but for guide purpose it has been done as separated method. Now we can merge this method into the class
+
+.. code-block:: python
+
+   class SlackReporter(Trigger):
+
+       def __init__(self, architecture, configuration):
+           Trigger.__init__(self, architecture, configuration)
+           self.slack_url = configuration.get("slack", "url")
+           self.channel = configuration.get("slack", "channel")
+           self.username = configuration.get("slack", "username")
+
+       def run(self, result, packages):
+           notify(result, self.slack_url, channel, username)
+
+Setup the trigger
+-----------------
+
+First, put the trigger in any path it can be exported, e.g. by packing the resource into python package (which will lead to import path as ``package.slack_reporter.SlackReporter``) or just put file somewhere it can be accessed by application (e.g. ``/usr/local/lib/slack_reporter.py.SlackReporter``.
+
+After that run application as usual and receive notification in your slack channel.