title: npm - save installed packages as dependencies
body: <p>I've installed some packages via <code>npm install $package</code>, without setting up a <code>package.json</code> first.
  Now I would like to create a <code>package.json</code> file, but keep all installed packages as dependencies.
  Simply running <code>npm init</code> doesn't offer this option, can I achieve this automatically?</p>