racters
  should be replaced by a
  special <a href="#option_specialCharPlaceholder">placeholder</a>.
  Mostly useful for non-printing special characters. The default
  is <code>/[\u0000-\u0019\u00ad\u200b\u2028\u2029\ufeff]/</code>.</dd>
  <dt id="option_specialCharPlaceholder"><code><strong>specialCharPlaceholder</strong>: function(char) → Element</code></dt>
  <dd>A function that, given a special character identified by
  the <a href="#option_specialChars"><code>specialChars</code></a>
  option, prod