title: filter option is not in ANTLR4
body: <p>I write <code>options{filter=true;}</code> in lexer grammar file and compile it with ANTLR4.
  It said </p>
  
  <pre><code> unsupported option 'filter'
  </code></pre>
  
  <p>It seems 'filter' only works with ANTLR3 </p>
  
  <p>I checked wiki <a href="http://www.antlr.org/wiki/display/ANTLR4/Options" rel="nofollow">http://www.antlr.org/wiki/display/ANTLR4/Options</a> and can not find the answer.
  'filter' key word  is not in the source file
  <a href="https://github.com/antlr/antlr4/blob/master/tool/src/org/antlr/v4/tool/Grammar.java" rel="nofollow">https://github.com/antlr/antlr4/blob/master/tool/src/org/antlr/v4/tool/Grammar.java</a>
  either.</p>
  
  <p>How could I fix this？</p>