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.
I did use code snippets from Jekyll (Tom's copyright) and jekyll-mentions (GitHub's copyright), so I included them in the license file. Should I move the license for the snippets somewhere else, or get rid of them entirely?