Support directives on Variable definitions

[rmosolgo]

See this update to the spec:

graphql/graphql-spec#486

At a minimum, we should update the parser so we don't choke on input like that. I don't have a plan for what to do with those directives, it can be considered in #1730 .

[rmosolgo]

Lol, never mind, it was quickly reverted in graphql/graphql-spec#492, so, we can keep our eyes out for it.