Migrate from alfredxing/jekyll-archives

Everything is pretty much done. I've set up tests with logic from [jekyll/jekyll-mentions](https://github.com/jekyll/jekyll-mentions), and everything seems to work!

The repo is also Travis-ready, so if you can set that up as soon as you'd like.
Oops, didn't realize `tags` was a Hash, though it seems this works as well. I'll fix this up.