title: Use Chokidar to watch for specific file extension
body: <p>I am looking to watch a folder using the nodejs Chokidar
  I only want to monitor for additions, deletes of xml files.
  I am new to Chokidar and can't figure it out.
  I tried setting the Chokidar ignore to match all strings that end in .xml but it looks like Chokidar ignore accepts negative regex's</p>
  
  <p>Even the below example doesn't work</p>
  
  <pre><code>watcher = chokidar.watch(watcherFolder, { 
      ignored: /[^l]$,
      persistent: true,
      ignoreInitial: true,
      alwaysState: true}
  );
  </code></pre>
  
  <p>Is there a way to do this or do I have to add the filter to the callback function?</p>
  
  <pre><code>watcher.on('add', function(path) {
      if (!/\.xml$/i.test(path)) { return; }
      console.log('chokidar: add: ' + path);
  });
  watcher.on('unlink', function(path) {
      if (!/\.xml$/i.test(path)) { return; }
      console.log('chokidar: unlink: ' + path);
  });
  
  watcher.on('change', function(path) {
      if (!/\.xml$/i.test(path)) { return; }
      console.log('chokidar: change: ' + path);
  });
  </code></pre>