title: What is --no-site-packages in virtualenv
body: <p>I am very new to python. I am trying to install virtualenv and create a sample project. 
  I could see the option like <code>--no-site-packages</code>. I tried(<code>virtualenv myapp</code>) without specifying option and try to install flask package(<code>pip install flask</code>). It installs in the package in <code>lib/python2.7/site-packages/flask</code>.</p>
  
  <p>What is the purpose of option <code>--no-site-packages</code>.</p>
  
  <p>PS: Virtualenv version - <code>1.11.2</code></p>