Add highlight around inline nicknames

Just something I was playing around with.

This makes inline nicks look a lot more like they do in Hipchat and other "rich" chat interfaces, by, specifically, giving them a border and light background.

![image](https://cloud.githubusercontent.com/assets/168193/4775004/4f864b26-5bb3-11e4-8116-b8c9ebc66a2e.png)