Refactor OpenGraph class to use one internal function `og_tag` for output.
Add support for OpenGraph publication date, last modified date, tags and categories, mostly because [Pinterest added support for article tags](http://blog.pinterest.com/post/62158724907/find-and-save-great-reads-with-new-article-pins).
Prevent outputting `[&hellip;]` by replacing it with `&hellip;`