Plugin: auto-exclude unused variables from request

Right now GraphiQL will send the full value of the `variables` form, even if they query itself doesn't make use of them. This can be pretty cumbersome on e.g. Hasura where it'll refuse to run a query if it sees unexpected variables present, but you might have multiple operations with their own variables and you want to be able to run them in quick succession.

It'd be nice if, just like auto-setting an `operationName`, GraphiQL plucked only the relevant variables to be sent to the server.