Add support for `@override` directive

[Ackar]

Add support for an @override directive as mentionned in https://github.com/APIs-guru/graphql-faker/issues/17#issuecomment-487934463

This works by finding all the fields with an @override directive in the extension schema and removing them from the source schema.

[hotsezus]

This PR solves new directive problem but i'm not sure if editor works properly when extending remote type and overriding some field. We need to be sure editor will not output that local field is already defined in remote schema.

[Ackar]

@AlexDerbenew It does work properly overriding remote types fields.

For example, running graphql-faker --extend http://swapi.apis.guru we can define the following extension schema:

extend type Film {
  title: String @override @fake(type: words)
}

[Ackar]

@IvanGoncharov Any thoughts on this?

[NShahri]

@IvanGoncharov is there any plan to merge or support this feature?

[moimikey]

this is the only thing stopping me from continuing to use this lib. @override is needed.