Extract open graph methods from Goldencobra::Article to Goldencobra::Metatag

Closed the last Pull Request and openend a new one. The last one had some flaws.

This change replaces the implementations of the opengraph related methods inside Goldencobra::Article with calls to new class methods on Goldencobra::Metatag. Both OpenGraph methods in Goldencobra::Article created Goldencobra::Metatag objects so it makes no sense to keep that code inside the Article class. Furthermore the methods were optimized to make them more readable and easier to understand.