title: NERDTree opens by default if opening a directory
body: <p>If I open a folder in vim like this:</p>
  
  <pre><code>$ mvim . # or vim .
  </code></pre>
  
  <p>NERDTree opens by default in full width:</p>
  
  <p><img src="https://i.stack.imgur.com/qYnBA.png" alt="enter image description here"></p>
  
  <p>How can I prevent this from happening and show default VIM welcome screen instead?</p>