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..