ListType with DictType

Currently it is not possible to have list of dicts. This is not working:

    class NewsModel(BaseModel):
        comments = ListType(DictType, field=StringType)

Because ListType expects `field` arg, and DictType also expects `field` arg. As a result, conflict of arguments.

This issue tries to fix it by applying different name for second field arg:

    class NewsModel(BaseModel):
        comments = ListType(DictType, nested_field=StringType)