Add "Edit this page" link

Stole the idea from Tom Black:
https://github.com/blacktm/standard-jekyll-template
https://github.com/blacktm/standard-jekyll-template/blob/gh-pages/_config.yml
https://github.com/blacktm/standard-jekyll-template/blob/gh-pages/_layouts/template/main.html
Pull upstream changes into _data/private
Push "Edit this link" into the edit_link include
Merge pull request #4 from 18F/editable-pages

Editable pages