Open graph fully absolute urls support

- Fixes the issue https://github.com/hexojs/hexo/issues/942
- Allows using fully absolute urls for og image tags (which is mandatory for Facebook).
- Fixes [poor external link detection](https://github.com/guiohm/hexo/commit/b424e11bfd3fd13f5978439eca545f45dbf547af#diff-62e9cf90d8889a8a3819d3cd1341272dL25)

I don't quite understand why the optional subdirectory had to be added on both `config.url`and `config.root`.
I believe this is more relevant now.