Registers a flag whose value is a whitespace-separated list of strings.

Any whitespace can be used as a separator.

name str, the flag name.
default list|str|None, the default value of the flag.
help str, the help message.
comma_compat bool - Whether to support comma as an additional separator. If false then only whitespace is supported. This is intended only for backwards compatibility with flags that used to be comma-separated.
flag_values FlagValues, the FlagValues instance with which the flag will be registered. This should almost never need to be overridden.
**args Dictionary with extra keyword args that are passed to the Flag init.

a handle to defined flag.