allow connection nodeType to be an interface

[jamiehodge]

Assuming that this does not violate the connection spec, allow the nodeType to be of type GraphQLInterfaceType.

Addresses: #160

I was somewhat confused by the existing connection spec. Would someone please advise regarding testing?

I'm mentioning @leebyron as well, to confirm that this doesn't violate the intent of one of the various GraphQL specs.

[jamiehodge]

@leebyron @wincent ?

[taion]

This should be an import type anyway, regardless of whether it's appropriate.

[yongwee]

Hi @jamiehodge do you plan to go ahead with this? I'm running into the same problem on my end 😅
Also, what about including GraphqlUnionType too?

[jaydenseric]

It can be pretty much anything except a list:

This field must return either a Scalar, Enum, Object, Interface, Union, or a Non‐Null wrapper around one of those types. Notably, this field cannot return a list.
— https://facebook.github.io/relay/graphql/connections.htm#sec-Node