Common interface for Element and Elements

[jevonmao]

It will be really convenient if SwiftSoup implements something like a parent class or protocol that encapsulates common methods between Element and Elements like .attr(). Right now I have to copy paste a lot of redundant code, even thought the method call and signature are exactly the same to handle Element and Elements.

[aehlke]

I would merge a PR for this if you want to do it. Thanks