title: Elegant way to check for missing packages and install them?
body: <p>I seem to be sharing a lot of code with coauthors these days. Many of them are novice/intermediate R users and don't realize that they have to install packages they don't already have.</p>
  
  <p>Is there an elegant way to call <code>installed.packages()</code>, compare that to the ones I am loading and install if missing?</p>