Separate variables in subscription

[boxmein]

What happens

Qminder.graphql.subscribe(query) does not accept a second, optional argument for variables.

What should happen

Qminder.graphql.subscribe(query, variables) should accept a second, optional argument variables which it passes to the GQL_START message and sends to the backend: https://github.com/apollographql/subscriptions-transport-ws/blob/master/PROTOCOL.md#gql_start

Why

• It's best practice to send variables next to the query document, instead of injecting them with string interpolation
• Prevents GraphQL injections during runtime
• Makes writing complex or parameterized functions clearer
• Helps utilize modern tooling such as graphql-tag which disallows string interpolation into queries