Set any arbitrary meta fields onto the meta instance

[jkimbo]

Following on from #1007

This PR sets all extra Meta class fields on the _meta instance. Currently any extra fields on the Meta class are lost. This PR keeps them around so that they can be used by application code.

Examples of why this is useful:

1. If you wanted to create some custom checks that all DjangoObjectTypes are using the fields meta option it's not currently possible. After this PR you could create this function:

def enforce_only_fields(schema):
	types = schema.get_type_map()
	for _, t in types.items():
		if issubclass(t.graphene_type, DjangoObjectType):
			assert t.graphene_type._meta.fields

2. You could implement field level permissions in Graphene Django by setting a default resolver and defining some kind of auth configuration like this:

    def authed_resolver(attrname, default_value, root, info, **args):
        auth_map = info.parent_type.graphene_type._meta.auth 
        if attrname in auth_map:
            perm_checks = auth_map[attrname]
            checks = [check(root, info) for check in perm_checks]
            if not all(checks):
                return None
        return graphene.types.resolver.default_resolver(
            attrname, default_value, root, info, **args
        )

    def staff_required(obj, info):
		if not info.context.user.is_staff:
			return False
        return True

    class AuthedType(DjangoObjectType):
        email = graphene.String()

        class Meta:
            model = Reporter
            default_resolver = authed_resolver
            auth = {
				"email": [staff_required]
			}

[jkimbo]

Discussed with @syrusakbary on Slack and I'm going to close this PR because this can better be solved in specific cases in Graphene Django:

Meta is intentionally “type safe”, I don’t think should be used as a bag for any kind of info.
The same can be achieved by creating another class inside of the Graphene Type

   class AuthedType(DjangoObjectType):
       email = graphene.String()
       class Meta:
           model = Reporter
           default_resolver = authed_resolver
       class MetaAuth:
           email = [staf_required]