title: plotRGB add title
body: <p>I would like to add a title to a plotRGB output.</p>
  
  <pre><code>#create random raster stack
  r1 &lt;- raster(nrow=5, ncol=5)
  values(r1) &lt;- 1:25
  
  r2 &lt;- r3 &lt;- r1  
  RGB_stack &lt;- stack(r1, r2, r3)
  </code></pre>
  
  <p>I have tried:</p>
  
  <pre><code># Create an RGB image from the raster stack
  plotRGB(RGB_stack, r = 1, g = 2, b = 3,
          axes=TRUE, main="My Title")
  
  
  plotRGB(RGB_stack, r = 1, g = 2, b = 3,
          axes=TRUE, addfun="(title='Test title')")
  </code></pre>
  
  <p>I have tried to turn the axes on and off. I am not sure what else to try to make this work. </p>
  
  <hr>
  
  <p><strong>UPDATE:</strong> based upon feedback below i figured out that </p>
  
  <pre><code>#Create an RGB image from the raster stack
  plotRGB(RGB_stack, r = 1, g = 2, b = 3,
          axes=TRUE, main="My Title")
  </code></pre>
  
  <p>works IF you have cleared out previous plots that might have the axes turned off. However i'd still like the plot to be an image with no axes ticks and just a title. </p>