title: Adding global title to Plots.jl subplots
body: <p>I would like to add a global title to a group of subplots using Plots.jl. </p>
  
  <p>Ideally, I'd do something like:</p>
  
  <pre><code>using Plots
  pyplot()
  plot(rand(10,2), plot_title="Main title", title=["A" "B"], layout=2)
  </code></pre>
  
  <p>but, as per the <a href="https://juliaplots.github.io/attributes/" rel="noreferrer">Plots.jl documentation</a>, the <code>plot_title</code> attribute is not yet implemented:</p>
  
  <blockquote>
    <p>Title for the whole plot (not the subplots) (Note: Not currently implemented)</p>
  </blockquote>
  
  <p>In the meanwhile, is there any way around it?</p>
  
  <p>I'm currently using the <code>pyplot</code> backend, but I'm not especially tied to it.</p>