Support 'converter' in field definitions

[smarie]

A few requirements/ideas:

• both as argument and as decorator. both can be specified. Decorator is @<field_name>.converter(*types) will be handled in @<field>.converter decorator to add a converter to a field #28
• if argument: a possibly ordered dictionary of type: converter or (*types): converter.
  - For old python support, a list of key/val tuples is supported.
  - '*': <> can be used to indicate "the rest"
  - None should be used for the none type
  - Is(x) for constant-only converters
• if argument: a single converter can be passed, in which case it will be considered '*'
• if several converters match, they will be tried in order until no exception is raised / no CONVERSION_FAILED is returned (returning None is considered a successful conversion)
• matching converters will be called even if received value is of a valid type, except if a specific option skip_matching_converters_if_valid=True is set now in Fast route without trying converters #30
• by default after conversion, validation will happen as always. A validate_after_converting=False argument or converter option would also be needed to change this behaviour. now in Ability for a converter to disable validation if conversion is successful #29

[smarie]

The spec moved a little bit while I was implementing. See doc for details.