use textContent to get the contents of the code element

this supports the use of character entities such as &lt; and &gt;

it's also the same technique used by highlight.js in the highlightBlock function https://github.com/isagalaev/highlight.js/blob/08062ea1c32f5a06b2b877ae2b4812d12f9355f1/src/highlight.js#L508
Merge pull request #23 from mikec/master

use textContent to get the contents of the code element