Add type based validation for flag based args

[cameronmore]

Functional Requirement:

• Allow users to define an expected type for any flag-based argument such as: —output Output where Output can be a string, or —number 15 where 15 must be an i32, and so on)
• Add this type to the help message, like:

-o, —output STRING  the output location
-n, —num     INT    the number of items to process

Implementation:

• Add as a possible ‘clean up’ task for the argManager.process() method?