Added a config option for setting a redirect prefix that ...

...is added to every redirect URI. Fixes #17.

While this fixes the linked issue, it doesn't yet fix the problem that you still need to hardcode the project name into the redirect prefix.

It'd be much better to use ```site.github.url``` for that, so my question: should I add this to the  ```redirect_url``` method, as in checking if the key exists and using that as the prefix? Or is there a better way?

/cc @parkr


I'm not a Ruby developer and you suggested that in https://github.com/jekyll/jekyll-redirect-from/pull/26#discussion_r11798216 so I thought it was fine, but I can change it of course :smile: