Support Query Variables

[ashkan18]

So we can do things like

query invoiceStates($ids: [ID!], $partner_id: String!) {
      invoices(ids: $ids, partner_id: $partner_id)
      {
        id
      }
}

[dblock]

I think this is the next important thing. Basically what is the equivalent of

          post '/api/graphql', params: {
            variables: {
              input: {
                email: 'matt@example.org'
              }
            },
            query: %{
              mutation($input: createRsvpInput!) {
                createRsvp(input: $input) {
                  id
                  total_count
                }
              }
            }
          }