EPIC: Directive support

Here to track the status of directives

:warning: these features are subject to change - if you are using any of these features make sure you have tests on your directives that actually run queries against a graph. :warning:

Execution directives
====
location | implemented | obj is
-----|----- | ---
QUERY | :heavy_check_mark: https://github.com/99designs/gqlgen/pull/744 | *ast.OperationDefinition
MUTATION | :heavy_check_mark: https://github.com/99designs/gqlgen/pull/744 | *ast.OperationDefinition
SUBSCRIPTION | :heavy_check_mark: https://github.com/99designs/gqlgen/pull/744 | *ast.OperationDefinition
FIELD | :heavy_check_mark: https://github.com/99designs/gqlgen/pull/756 | parent type
FRAGMENT_DEFINITION | :no_entry:
FRAGMENT_SPREAD | :no_entry:
INLINE_FRAGMENT | :no_entry:

Schema directives
===
location | implemented | obj is
---|--- | --
SCHEMA | :no_entry:
SCALAR | :no_entry:
OBJECT | :heavy_check_mark: https://github.com/99designs/gqlgen/pull/1117 | parent type
FIELD_DEFINITION | :heavy_check_mark: | parent type
ARGUMENT_DEFINITION | :heavy_check_mark: https://github.com/99designs/gqlgen/pull/460 | argument bag
INTERFACE | :no_entry:
UNION | :no_entry:
ENUM | :no_entry:
ENUM_VALUE | :no_entry:
INPUT_OBJECT | :no_entry:
INPUT_FIELD_DEFINITION | :heavy_check_mark: | parent input

Schema field directives on subscriptions
===
obj is always nil for any root (Query, Mutation, Subscription)
next() returns a channel of the field type