title: Plot on ggmap by using city and state
body: <p>I have a map of the USA, ready to go:</p>
  
  <p><code>mygggmap &lt;- get_map(location = "United States", zoom = 4)
  g &lt;- ggmap(myggmap)</code></p>
  
  <p>The next step is to plot some major cities.</p>
  
  <p>I understand I can create a layer to plot my major cities by their longitude and latitude, something like:<br>
  <code>g &lt;- g + geom_density2d(data = citydata, aes(x = lon, y = lat))</code></p>
  
  <p>but I am wondering if there is a way to plot by the actual name, city and state?  Since ggmap is layered over the Google Map API, this should be possible, correct?</p>