Being able to parse without validation during init

[GMZwinge]

Is your feature request related to a problem? Please describe.
In our init, we sometime set some flag's default based on parsed flags. For example, when moving data between servers, we use the values of some flags for the first server as defaults for the other server(s). It is currently possible, except for required flags where the parse fails the validation.

In some cases where multiple command are for a type of environment, we have a base class shared by those commands. In that base init, it would also be nice to do a parse without validation failure for the flags in the derived classes.

Describe the solution you'd like
It would be nice to be able to parse without validation.

Describe alternatives you've considered
Our current workaround is to save the required values, reset the required values, parse and restore the required values.

Additional context
Related to #854 and #829.

[AllanOricil]

#1507 would solve this issue as all validations would then be decoupled from flag definitions