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.
No, it has to be in `site.config` to be output as `{{ site.archives }}`. Otherwise you have to do monkey patching and the like.