Add reference links support

Hi,
This commit tries to resolve #184.
I really want to have this feature so I decided to implement it by myself.

Known issue: the content in the `<meta name="description" content="">` tag would show "[text][id]" literally, instead of transforming it into "text".
Because _.toArray(section) destroys section.links, these two can't work.
There might be a better solution though. As _.toArray is only used to copy the section, it might be possible to copy it and preserve .links using other method.