Allow custom resolvers in options

[jneurock]

While a somewhat custom resolver can be created with this addon by mapping a field to a callback, this isn't quite the same as a resolver. Allowing the developer to include their resolvers in the options opens up some flexibility and provides a way for them to include resolvers they may have already created. It could also be the case that developers want to work more with resolvers since their knowledge of the subject is portable.

[jneurock]

This addon will be deprecated in favor of https://github.com/miragejs/graphql which does allow optional resolvers.