FuncBoolVar is a type of flag that accepts a function, converts the user's value to a bool, and then calls the given function.

FuncDurationVar is a type of flag that accepts a function, converts the user's value to a duration, and then calls the given function.

FuncOptionalStringVar is a flag that accepts a function which it calls on the optional string given by the user.

FuncVar is a type of flag that accepts a function that is the string given by the user.

StringFlag implements the flag.Value interface and allows multiple calls to the same variable to append a list.