The first call to Set will clear the map before adding entries;
subsequent calls will simply append to the map.
This makes it possible to override default values with a command-line
option rather than appending to defaults,
while still allowing the distribution of key-value pairs across
multiple flag invocations.
For example: `--flag "a:hello" --flag "b:again" --flag "b:beautiful"
--flag "c:world"` results in `{"a": ["hello"], "b": ["again",
"beautiful"], "c": ["world"]}`
Kubernetes-commit: 6e49ac382bbd43e5a3f648607f8005bdf0237317
Michael Taufen