title: Nesting string format
body: <p>I'm writing some reports, and I'd like to know if there's a simpler way to obtain the following behavior.</p>
  
  <pre><code>&gt;&gt;&gt; '{:-^60}'.format('Percentage used: {:.2%}'.format(.4))
  '------------------Percentage used: 40.00%-------------------'
  </code></pre>
  
  <p>As you can see I'm centering the text and then printing a number formated to percentage. But I'm using a format function inside another. If possible I'd like to do the same in a single function, kind of the following.</p>
  
  <pre><code>'$SOMETHING GOES HERE'.format(header = 'Percentage Used:',percentage = .4)
  </code></pre>
  
  <p>Of course I'm looking for a general solution, that would work with all, or most, of the formating options, not just to alignment</p>
  
  <p>Thanks.</p>