Add method for validating a parser

[jonsequitur]

In order to improve performance, a number of validations that were done while creating commands (e.g. alias collision checks on Command.Add) have been removed in beta 2. These checks are useful for developers but once the parser code is written, these checks are unnecessary performance overhead for the end user (unless the parser is assembled dynamically based for example on runtime configuration).

These validations should be opt-in via a public method that can be used for example during unit testing or during dynamic wire-up as appropriate.

[jonsequitur]

Resolved by #1582.