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

The flag value is parsed with a CSV parser.

name str, the flag name.
default list|str|None, the default value of the flag.
help str, the help message.
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.