Interfaces implementing other interfaces loose implements clause when --includeDirectives option set

[thillm-amazon]

The GraphQL spec supports interfaces implementing other interfaces:
graphql/graphql-spec#373

When using graphql-schema-utilities with the --includeDirectives flag set, the implements ... clause is removed from Interfaces in the merged schema.

Example

Input Schema

interface Node {
  id: ID!
}

interface FooNode implements Node {
  id: ID!
  foo: String
}

Output Schema from graphql-schema-utilities with the --includeDirectives flag set

interface Node {
  id: ID!
}

interface FooNode {
  id: ID!
  foo: String
}

[thillm-amazon]

Fixed by #51