Validatable option parameters

[grkvlt]

The parameters for an option should be validatable (beyond the allowed options property). This could be done by accepting a class name for a Predicate which tests if they are acceptable. Some standard predicates could allow things like regular expressions. For example:

@Option(name = "-v", validator = CheckValid.class)
String v;

[dain]

The plan is to use Java Bean Validation Framework (http://bval.apache.org/) for command validation. The real issue is if we want this to be core to the CLI framework or an add-on. The main issue is people usually have different ideas on how to report errors to the user.