Support for sorting by relationship attributes

Seems like this isn't supported right now.

The spec says:

Note: It is recommended that dot-separated (U+002E FULL-STOP, ".") sort fields be used to request sorting based upon relationship attributes. For example, a sort field of author.name could be used to request that the primary data be sorted based upon the name attribute of the author relationship.

Will see if I can whip up a PR.

[davidgoli]

👍 also for filtering

[aldhsu]

Closed by #494