Add support for configuring parameters

[AlexGhiondea]

When using parameters, sometimes you would need to tweak them based on some rules.

One example is the use of the output file. The user will provide a value for that, but what happens if the file already exists?

An approach is to automatically append a number to it until it creates a file name that no longer exists.

Other validations could be added -- what if a parameter can have a min/max value? That can be validated by the parser.

Couple of approaches:

• Attributes that describe the behavior
• Callbacks that are registered with the parser for validation