`author` tag that drops in author info on demand

This PR allows us to use an author tag like so:

![neat2](https://cloud.githubusercontent.com/assets/4592/4344307/6b0fb4ee-4089-11e4-9ca8-bb957b260ec1.png)

To generate whatever stuff we want for that author out of `_data/team.yml`, like maybe images:

![neat](https://cloud.githubusercontent.com/assets/4592/4344309/740b9586-4089-11e4-8406-9a09685a2f47.png)

**This PR does not add the avatars above.** It adds just the `<span>` around the name, and looks identical to before. This PR retains:

![now](https://cloud.githubusercontent.com/assets/4592/4344321/c5faace2-4089-11e4-9dfd-2bac00373ed9.png)

Though, I did change the `<p><em>` to a `<p class="author">` in one blog post, and kept the existing style.

Note: this is **downstream** from #203, which should be merged first.