add WithIgnoreUndefined option to config parse

[zackattack01]

Currently, if an unknown option is found in our launcher.flags the parser will stop parsing and return an error which is not checked/logged. This adds an option to the parser to prevent this behavior and continue parsing, discarding the unknown flag

[directionless]

@RebeccaMahany Would this have eliminated the need for github.com/spf13/pflag ?

[RebeccaMahany]

@directionless -- I don't think so. Unless the behavior has changed -- the last time I looked into it when I ultimately added pflag instead, WithIgnoreUndefined worked for flags supplied via config file, but would still break for us with undefined command-line args.

Separately, I am not sure if this PR will address #1513 -- older launcher installations without this option set will still run into this issue.