title: Using the babel-register `only` option
body: <p>I have the following config in my .babelrc:</p>
  
  <pre><code>{
      "presets": ["react", "es2015"],
      "only": ["app/**/*.js", "src/**/*.js", "test/**/*.js", "node_modules/my-module/**/*.js"]
  }
  </code></pre>
  
  <p>I basically want my-module in the node_modules directory to be passed through babel, and so am using the <code>only</code> option.</p>
  
  <p>However, it doesn't seem to be working. Any ES6 I write—whether in my-module or even just app/app.js—isn't transpiled, and so breaks my app.</p>
  
  <p>Any ideas? Am I misunderstanding the documentation?</p>
  
  <p>Previously I was doing the following:</p>
  
  <pre><code>require('babel-register')({
      ignore: function (filename) {
          if (filename.indexOf('node_modules') === -1) {
              return false;
          }
  
          return filename.indexOf('@lostmyname/styleguide') === -1;
      }
  });
  </code></pre>
  
  <p>However, I need to use the .babelrc because I'm adding mocha, which only supports the require hook with no options.</p>