Add support for `profile` attribute for links (other attributes?)

[kookster]

In our API we use the profile attribute of links extensively, but they are not available from the HyperResource::Link instances. It would be great to add this as an attribute for that class.

That also makes me wonder though about other link relation attributes that might be on the link. Should there be a way for any attribute of a link to be available, perhaps an attributes hash that has whatever was passed in to create the HyperResource::Link?