Adding facebook social comment plugin configuration

Hi..

I wrote facebook social comment plugin partial template to hexo-theme-light.

The configurations are combined by two sections:

* comment_provider - comment provider name. now "facebook" allows.
* facebook - facebook social plugin settings
** appid - facebook appid
** comment_count - max social comment counts
** comment_width - comment view layout width by pixel
** comment_colorscheme - comment view color scheme (refrenced by https://developers.facebook.com/docs/plugins/comments/)

Please test and consider merge codes.
Thank you..