title: filled.contour - delineate the change in the z value levels
body: <p>I have to produce <code>filled.contour</code> plots that have some kind of symmetry in the z-values like in the below code</p>
  
  <pre><code>x &lt;- 1:5
  y &lt;- 1:5
  z &lt;- matrix(outer(x,y,"+"),nrow=5)
  filled.contour(x,y,z)
  filled.contour(x,y,z,color.palette=rainbow)
  z2 &lt;- z
  z2[5,5] &lt;- Inf
  filled.contour(x,y,z2,col=rainbow(200),nlevels=200)
  </code></pre>
  
  <p>I would like to be able to delineate the levels of the z value for example certain levels or increments (2,4,6,8,10) increments so as to obtain the following plot</p>
  
  <p><img src="https://i.stack.imgur.com/n5vU6.png" alt="enter image description here"></p>
  
  <p>I want to leave the levels of <code>col=rainbow(200)</code> in order to plot smooth colors and color transitions but I want to delineate certain values/increments. </p>
  
  <p>Is this possible? I would appreciate a base R solution but in case this is not possible any solution would be good (ggplot, lattice)</p>