keep .highlight css class

highlight area now gets two classes: `div.highlight.hl-language`

CSS header is generated for class `.highlight`, but highlighted divs were only given class `.hl-language`, so the CSS wasn't applying.

This alters the effect of PR #4682 from *changing* the highlight class to *adding* a highlight class.

Before:

![screen shot 2014-04-29 at 16 17 04](https://cloud.githubusercontent.com/assets/151929/2836067/fdfa1462-cff4-11e3-9e29-9aec4122fd6f.PNG)

After:

![screen shot 2014-04-29 at 16 16 26](https://cloud.githubusercontent.com/assets/151929/2836069/03b3a44a-cff5-11e3-97f9-b7b2380f1d47.PNG)