title: Less mixin with optional parameters
body: <p>I have a <code>Less</code> mixin defined as:</p>
  
  <pre><code>.fontStyle(@family, @size, @weight: normal, @style: normal, @color: #ffffff, @letter-spacing: normal) {
    font-family: @family;
    font-size: @size;
    color: @color;
    font-weight: @weight;
    font-style: @style;
   letter-spacing: @letter-spacing;
  }
  </code></pre>
  
  <p>How can I define usage like:</p>
  
  <pre><code>.fontStyle('NimbusSansNovCon-Reg', 12px, , , , 0.1em);
  </code></pre>
  
  <p>I.E. use the defaults for <code>@weight</code>, <code>@style</code>, <code>@color</code></p>