title: Run code before all Airflow DAGs
body: <p>I'm using the relatively new <a href="https://github.com/apache/incubator-airflow" rel="nofollow noreferrer">airflow</a> project. I have a bunch of DAGs written and running. Now I want to integrate a bug reporting service, so that if any code in any of the DAGs raises an exception, the information will be sent to a certain API. I can put the API call in the <code>on_failure_callback</code> of each DAG, but I need to execute an initializing line like <code>bug_reporter.init(bug_reporter_token)</code> that just needs to run once. </p>
  
  <p>Is there a place in Airflow for initializing code? Right now I'm initializing the bug tracker at the beginning of every DAG definition file. This seems to be redundant, but I can't find a place to write a file that runs before the DAGs are defined. I've tried reading about <a href="https://airflow.incubator.apache.org/plugins.html" rel="nofollow noreferrer">plugins</a>, but it doesn't seem to be there.</p>