Include 'linked' member for json-api collections

Include top-level 'linked' member when using JSON-API adapter. Needed to pass the hash to the elements in the collection, using the options hash worked, but I'm open to suggestions.  
Initially, I believed it should be `include: [:author, :comments]` instead of strings because we need to whitelist these types, then we'd have to call `.join(",")` again.

However, we also need to support `include=comments,comments.author` (sublevels), which would invalidate the idea with symbols above. I don't know how we should express that, though. Perhaps `include: [:author, [:comments, :author]]`? Do you have any idea?