Adds an RSS feed link to the follow section

This adds an RSS feed link and icon to the Follow Us box underneath every blog post:

![follow-us](https://cloud.githubusercontent.com/assets/4592/5041588/aba2a3d8-6b93-11e4-83c3-eec3398c3b39.png)

There's an extra pixel I don't like, because I'm hacking my way into the existing setup, without regenerating the custom 18F font, by using a dot, `.` as the `::before` content for the icon. It blends in, but you can see the extra pixel.

Input welcome! Seems worth advertising our news feed, since we have one.