Implement a Tab component that can be used across the different websites

[mansona]

As part of the discussion on ember-learn/ember-website#530 we identified that we should probably move the tab implementation into the styleguide.

I think that is a good idea but we should implement the styleguide tabs component in collaboration with someone working on another Ember website (such as the api docs) so that the API can be discussed and finalised in such a way that is flexible enough for all websites and makes sure the designs are cohesive 👍