Feature request: meta field on relationships

[dpikt]

Per the spec v1.0 (emphasis added):

A “relationship object” MUST contain at least one of the following:

links: ... data: ... meta: a meta object that contains non-standard meta-information about the relationship.

Right now, serializer relationships support the first two options (https://github.com/Netflix/fast_jsonapi#links-on-a-relationship) but not the last.