POD-based Option Value Validation

Bug 821684 highlights the need for something we've discussed before: POD-based option value validation. Many options are like:

type: string; default value: foo

Then they have other possible values like "bar", etc. Currently, we validate these with special, non-uniform code--or at least, we usually do, and that's the problem. Some tools accept invalid values and the results can be bad. (E.g. throttling isn't enabled and the tool causes slaves to lag).

It would be nice if OptionParser could validate values for such options based on directives in the POD.

Related: http://code.google.com/p/maatkit/issues/detail?id=405