Undictify

- Allow target values of type Any to accept dicts.

- Disallow conversion when Unions are used as target.
- Improved exception messages.

- Support for unions made of built-in types
- Support for dictionaries as parameters in target functions

Add support for meaningful conversions from string to bool (with `convert==True`):
- `True <- ['1', 'yes', 'true', 'on']`
- `False <- ['0', 'no', 'false', 'off']`

minor Pylint fix

Add support for omitting function parameters with default value