Interface handling: generate queries for interface types + FRAGMENT_TYPE fix

[flazoon]

This set of changes addresses the following items:

1. Generate queries for interface types (but not mutations).
   e.g.

interface Person {
userId: ID!
name: String
}
type User implements Person {
userId: ID!
name: String
rated: [Rated]
}

type Query {
Person(
userId: ID
name: String
_id: String
first: Int
offset: Int
orderBy: [_PersonOrdering]
filter: _PersonFilter
): [Person]
User(
userId: ID
name: String
_id: String
first: Int
offset: Int
orderBy: [_UserOrdering]
filter: _UserFilter
): [User]

}

2. Fix an issue with FRAGMENT_TYPE value picking the first available label.
   The fix follows the solution suggested here: Filter out labels with underscores. #301 (comment)

[johnymontana]

Hey @flazoon! Thanks so much for this PR! Would you mind pulling in the latest commits from master? Looks like we had a slight conflict with another PR.

But otherwise, this looks good to go!

[flazoon]

Thanks @johnymontana for the heads up, I've merged latest and fixed the conflicts.

[michaeldgraham]

This looks awesome @flazoon, thank you so much for your work on this! I'm sorry about the timing with PRs here - I hope the conflicts aren't too much of a hassle 🎉

[flazoon]

My pleasure to contribute @michaeldgraham. Always glad to see refactoring changes.

[johnymontana]

Perfect. Thanks @flazoon!