title: continent outlines in R
body: <p>is there a way to plot only the continental outlines of rworldmap in R (no countries, no data)?  </p>
  
  <p>There are two other questions close to this one, but they are to fill the continents with colors, and the commands are to plot data. 
  <a href="https://stackoverflow.com/questions/20146809/how-can-i-plot-a-continents-map-with-r">How can I plot a continents map with R?</a></p>
  
  <p>I only want to overlay the continental outlines without country lines and without requiring other data.</p>