Allow mention from a different base url than https://github.com

Jekyll gives the generators the configs, not the site

https://github.com/jekyll/jekyll/blob/e6698fc65ae9d26d2f5566cb4270c2d628
645b8d/lib/jekyll/site.rb#L391
Add tests
Fix the tests.
Merge pull request #14 from jekyll/mention-from-twitter