title: Cache maps using ggmap
body: <p>I'm plotting maps using the <code>ggmap</code> package. To download a map over the internet, I can use this code:</p>
  
  <pre><code>library(ggmap)
  get_map(location = c(-1.81, 55.655), zoom = 12, maptype = "hybrid")
  </code></pre>
  
  <p>Is there a way to avoid downloading maps over the internet, and instead import a .png file from a local folder? Or in other words, download maps once, cache the .png and thereafter import the .png from a local folder? My connection is rather slow, I constantly redownloading the same base map is wasting valuable time.</p>