Allow multiple values for "rel"

[handrews]

Addresses #350

This follows the precedent of "type" by allowing "rel" to be
either a single string, or an array of strings.

Also paging @mokkabonna

[philsturgeon]

What is the use-case here?

[handrews]

@philsturgeon multiple rel values are well established in RFC 5988/8288 (the theoretical model and the HTTP Link header serialization). They are also well established in the HTML <link /> element, and while I reject analogies with HTML that limit Hyper-Schema, I do think that the non-presentation aspects of HTML are a good reference for a minimum baseline functionality.