title: Airflow - how to make EmailOperator html_content dynamic?
body: <p>I'm looking for a method that will allow the content of the emails sent by a given EmailOperator task to be set dynamically.  Ideally I would like to make the email contents dependent on the results of an xcom call, preferably through the html_content argument.</p>
  
  <pre><code>alert = EmailOperator(
      task_id=alertTaskID,
      to='please@dontreply.com',
      subject='Airflow processing report',
      html_content='raw content #2',
      dag=dag
  )
  </code></pre>
  
  <p>I notice that the Airflow docs say that xcom calls can be embedded in templates.  Perhaps there is a way to formulate an xcom pull using a template on a specified task ID then pass the result in as html_content?  Thanks</p>