title: usepackage and making macros in ipython notebook
body: <p>How can user macros and packages be included in the latex rendered in markdown cells in ipython?</p>
  
  <p>e.g.:</p>
  
  <pre><code>\usepackage{amssymb}
  \newcommand{kms}{\ensuremath{\mathrm{km~s}^{-1}}
  </code></pre>