More meta tags and some refactoring

Hi!
I just came across you middleman plugin and decided to use it in a project since i am already familiar with the rails meta-tags gem.

As I need more than just title and description I added some stuff that I found here: https://www.iacquire.com/blog/18-meta-tags-every-webpage-should-have-in-2013.
Since I am no SEO guy ore something like that I do not know if that is really all cool, but it seems quite reasonable.

While adding more tags I also felt like refactoring the thing a bit towards more convenience for adding new tags. Please check it out and tell me if you like it.

Greetings,

prosac