title: Elasticsearch: JSON to get frequency of specific term
body: <p>I think this is a straightforward application, yet I cannot find a recipe on the internet.</p>
  
  <p>Can you suggest a <code>JSON</code> query to send through <code>python</code> to an Elasticsearch instance that would return the frequency of a specific term in a certain field?</p>
  
  <p>I guess it should be possible by some tweak of the Term Vector API, but it seems not straightforward.</p>
  
  <p>I would not mind to get both the absolute frequency and the number of document containing the term.</p>