Parameter post-converters

[epsy]

Currently Clize converts each Python parameter to a Parameter instance by calling a function and using its result:

• Either the first annotation on that parameter if it is a parameter converter,
• or the default converter

This makes it impossible to use parameters.multi with parameters.one_of, for instance. This is counter-intuitive and also quite unfortunate.

To fix this, the parameter converter should collect callables from the annotations designated as "post-converters". Once the parameter converter has completed creating a Parameter instance, it should post-process this parameter with each post-converter in order. Each post converters does some changes and returns a Parameter instance, much like decorators.

Example post-converters:

• parameter.one_of, parameter.mapped, (both already existing as parameter converters)
• argument_decorator, (already existing as parameter converter)
• Something to change the name of the value in the help (argparse calls it metavar)
• UNDOCUMENTED and LAST_OPTION could be refactored as post-converters.

The documentation will need additional clarification on the roles of parameter converters, post-converters and value converters, possibly with a drawing :)