title: Babel multiple directores into single output directory
body: <p>I've scouered the documentation for Babel and cannot seem to find an answer, so I turn to the glorious community.</p>
  
  <p>With a directory structure like this:</p>
  
  <pre><code>src/
      folder1/
          file1.js
          file2.js
      folder2/
          file3.js
      folder3/
          file4.js
          file5.js
  </code></pre>
  
  <p>I want Babel to transpile all files into a flattened directory:</p>
  
  <pre><code>lib/
      file1.js
      file2.js
      file3.js
      file4.js
      file5.js
  </code></pre>
  
  <p>No matter what I try, Babel always inherits the <code>src/</code> directory structure. Any ideas?</p>