Allow omit query_param and omit keyword

[robertcarr]

I would like to specify some fields to omit when I initialize the serializer and also allow the API request query_param to omit extrfa fields. Right now it's either or but merging the two would be nice. Also the same for fields, possibly.

For example:
This should return all the fields except password &email. Right now, it will only omit password.
FlexSerializer(data=request.data, omit=['password'])
GET /api/?omit=email

      def __init__(self, *args, **kwargs):
        expand = list(kwargs.pop("expand", []))
        fields = list(kwargs.pop("fields", []))
        omit = list(kwargs.pop("omit", []))

        super(FlexFieldsSerializerMixin, self).__init__(*args, **kwargs)

        self.expanded_fields = []
        self._flex_fields_applied = False

        self._flex_options = {
            "expand": (
                expand
                if len(expand) > 0
                else self._get_permitted_expands_from_query_param()
            ),
            "fields": (
                fields if len(fields) > 0 else self._get_query_param_value("fields")
            ),
            "omit": omit if len(omit) > 0 else self._get_query_param_value("omit"),
        }

[rsinger86]

This seems like a straightforward change to the source code, but I'm concerned that it may cause unexpected behavior in existing projects.