Make cli flagset name configurable

[mofirouz]

A small feature request.

It would be great to be able to make the flagset name configurable. Currently, it is statically set to xFlags.

Moreover, it would be great if we could append additional flags to the flagset without them appearing in the config struct. For instance, things like "verbose" logging that we only ever setup during the application start and affects the log level will never be used anywhere else in the application and are not useful to have in the config struct.

Potentially exporting the fields in FlagMaker?

Thanks a lot.