Add google-analytics processor

[ikalnytskyi]

So far Holocron followed approach when Google Analytics counter was a part of theme templates, hence the theme was responsible for providing a way to propagate user's counter into HTML.

While this approach's good by its intention, it requires all the theme to implement the same thing. And if, by some reason, the theme doesn't implement we have no way to use Google Analytics.

So this ticket suggest to implement google-analytics processor that injects counter code into documents, providing a way not only to use GA counter with third-party theme but injecting counter into any HTML document.