Add github repository to config.yml.

This fixes an incompatibility between with the github-metadata plugin and
people who have configured multiple github accounts via ssh host aliases
(e.g. using github.com-personal and github.com-usds when cloning a remote).
Without an explicit repo name, the github-metadata plugin requires an origin
url be exactly "github.com" or the command "jekyll serve" will fail.

_config.yml

@@ -15,6 +15,7 @@ url: "https://www.usds.gov" # the base hostname & protocol for your site
 twitter_username: usds
 github_username:  usds
 medium_url: https://medium.com/@USDigitalService
+repository: usds/website
 
 # Build settings
 markdown: kramdown