title: Title Heading for Tree R Package
body: <p>I want to add a main title heading to the plot output of a <code>tree</code> model.</p>
  
  <pre><code>library(tree)
  data(cpus, package="MASS")
  cpus.ltr &lt;- tree(log10(perf) ~ syct+mmin+mmax+cach+chmin+chmax, cpus)
  cpus.ltr
  summary(cpus.ltr)
  plot(cpus.ltr, main="test")
  text(cpus.ltr, main="test")
  </code></pre>
  
  <p>Alas, I do not seem to have a heading. </p>
  
  <p><img src="https://i.stack.imgur.com/9YE5g.png" alt="no heading :("></p>
  
  <p>How do I add a title? </p>