title: Formatting big.Rat using golang.org/x/text/message
body: <p>The package <a href="https://godoc.org/golang.org/x/text/message" rel="nofollow noreferrer"><code>golang.org/x/text/message</code></a> allows us to format numbers using national formats:</p>
  
  <pre class="lang-golang prettyprint-override"><code>const n = 1222333.444555
  
  prEn := message.NewPrinter(language.English)
  prEn.Printf("%20.6f\n", n)
  // Prints:
  //     1,222,333.444555
  
  prRu := message.NewPrinter(language.Russian)
  prRu.Printf("%20.6f\n", n)
  // Prints:
  //     1 222 333,444555
  </code></pre>
  
  <p>Can I use it with <code>math/big.Rat</code>? That is, something like (doesn't work):</p>
  
  <pre><code>rat := big.NewRat(1222333444555, 1000000)
  prEn.Printf("%20.6f\n", rat.FloatString(6))
  // Should print:
  //     1,222,333.444555
  </code></pre>
  
  <p>I know that I can wrap <code>Rat</code> in my own type and implement <a href="https://golang.org/pkg/fmt/#Formatter" rel="nofollow noreferrer"><code>fmt.Formatter</code></a>, but maybe there is a built-in way already?</p>