Underline auto-generated tag links

/cc @cantlin Sorry - forgot I fixed this last week and didn't raise a pull request. Blame hackday.

![anigif_enhanced-buzz-27927-1388776392-0](https://f.cloud.github.com/assets/84996/1983553/e5692d26-8417-11e3-8a15-b8b5833d52a7.gif)