title: Python how to update a dictionary with another dictionary by applying add to existing values
body: <p>I have an existing dictionary with string keys and numerical values. I create another dictionary containing new keys and values which I need to <code>update</code> over the existing dictionary. </p>
  
  <p>Only requirement is I need that if a key in new dictionary already exists in target dictionary, the value is added to existing value, instead of replacing it. </p>
  
  <p>How can I achieve this in Python 2.7?</p>