title: Title over group of subplots
body: <p>there are many subplots and each subplot has its own title. how can add a title over all of these group of subplots?
  I want this title to be shown at top center.</p>
  
  <pre><code>x = linspace(-5,5);
  
  y1 = sin(x);
  subplot(2,5,[1:2])
  plot(x,y1)
  title('y=sin(x)')
  
  y2 = cos(x);
  subplot(2,5,[3:4])
  plot(x,y2)
  title('y=cos(x)')
  
  y3 = tan(x);
  subplot(2,5,[5,10])
  plot(x,y3)
  title('y=tan(x)')
  
  y4 = sin(2*x);
  subplot(2,5,[6:7])
  plot(x,y1)
  title('y=sin(2x)')
  
  y5 = cos(2*x);
  subplot(2,5,[8:9])
  plot(x,y2)
  title('y=acos(2x)')
  </code></pre>