Add possiblity to specify default values for for record fields:

Say we want to derive:

    data GGResult = GGResult
      { _ggrtPartialMatch       :: Bool
      } deriving (Show, Typeable)

    deriveJSON
       defaultOptions
        { fieldLabelModifier = fromCamel 5
        , fieldsWithDefaults = [ ("partial_match", [| False |]) ]
        }
       ''GGResult

Now whenever parser was expecting field "partial_match" inside JSON file, will not fail but return a value `False`.