omit_resolver_fields also respects gqlgen.yml's config

[aereal]

This PR will fix the #3044.

The omit_resolver_fields option that introduced by #2957 does not respect models.<model>.fields.<field>.resolver option, only schema directives are respected.
The models.<model>.fields.<field>.resolver option marks the field to be resolved with explit resolvers, so the omit_resolver_fields should respect also this option.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 75.011% (-0.02%) from 75.029%
when pulling 0044392 on aereal:omit-resolver-fields-respect-gqlgen-config
into 769632a on 99designs:master.

[StevenACoffman]

Hey, how's this proceeding?