Add ability to exclude schema members from the generated docs

[Fs00]

Is your feature request related to a problem? Please describe.
When generating the docs for a graphql-ruby schema, the resulting documentation includes types used for introspection such as __Type, __Field, __Directive etc. which I'd rather not have in my API documentation.
Also, I might want to hide some mutations, fields or types in the docs because I use them for internal purposes and I don't want other clients to rely on them (I know that ideally I should hide them in the schema itself, but it's not always convenient or desirable).

Describe the solution you'd like
I envision an option similar to the notices one: a user-provided proc that, given the schema member path, returns true if the member should be hidden.
To satisfy the use-case described above, one could write the option as: (feel free to choose a different/better name for the option)

options[:hide_if] = ->(schema_member_path) { schema_member_path.start_with?('__') }

[brettchalupa]

@Fs00 Makes sense to me, thanks for logging this! Seems like a worthwhile improvement. Not sure if I'll be able to dig into it soon, but it's certainly open for others to take on if they want. 👍