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.
It's generally better just to write these things out to `destination` and add them to `site.keep_files`. If you think the general use case requires them in `site.pages`, then I'd rather keep them out of it.

Alternatively, you set it to `site.config['archives']` (if it's not set) so people can iterate over `{% for archive_page in site.archives %}` more explicitly. Either `archives` or `archive_pages` would work for this solution.

What do you think?