text: Title: Constructors for auto generated classes?
  Body: We're using sgqlc to interact with a GQL service we've built and love the removal of dict-of-stuff navigation in the response objects and field selections.  We can't quite figure out how to do the same for complex argument/input values though.
  As an example:
  ```python
  class FilterDateInput(sgqlc.types.Input):
      start = sgqlc.types.Field(String, graphql_name='start')
      end = sgqlc.types.Field(String, graphql_name='end')
  class FilterValueInput(sgqlc.types.Input):
      value = sgqlc.types.Field(String, graphql_name='value')
  class FilterInput(sgqlc.types.Input):
      date = sgqlc.types.Field(FilterDateInput, graphql_name='date')
      id = sgqlc.types.Field(FilterInput, graphql_name='id')
  ```
  Ideally we would be able to generate a filter by:
  ```python
  gql_filter = FilterInput(
      date=FilterDateInput(start='04-12-2012', end='04-15-2012'), 
      id=FilterValueInput(value='A17')
  )
  operation.queryThings(filter=gql_filter, user="bill")
  ```
  This would benefit from tool tips and autocomplete where as the current method we're aware of is:
  ```python
  gql_filter = { 
      'date': {'start':'04-12-2012', 'end':'04-12-2012'}, 
      'id':{'value':'A17'}
  }
  operation.queryThings(filter=gql_filter, user="bill")
  ```
  Which as the schema grows become increasingly unwieldy. Is there a way that we are missing to do this or is there a way to generate classes that will act in this way using introspection of the sgqlc’s autogenerated classes?
      