Add Custom Extractor sub-section

I added the custom extractor method to the website as we talked previously. My editor removed trailing whitespace, hope that you don't mind that.

Here is a screenshot of how it looks.
![screen shot 2014-05-09 at 21 49 16](https://cloud.githubusercontent.com/assets/625264/2932051/b870bce4-d7aa-11e3-8273-596ff2860641.png)