swagger: implement custom convertors

[sgratzl]

e.g. if it is a type=string with format=range it should be converted to a range automatically.

see also http://exploreflask.com/en/latest/views.html#custom-converters
and http://connexion.readthedocs.io/en/latest/cookbook.html#custom-type-format

range -> ranges.parse
json -> json.loads