Optional arg and switch

@L7R7 This changes the way parsing optional arguments work. I think this change is good, but this test now fails and I'm not sure if it's fixable: ``` argParseSpec ["--foo", "bar"] ((,) <$> setting [reader str, argument] <*> setting [reader str, argument]) ("--foo", "bar") ``` ``` opt-env-conf-test> test/OptEnvConf/RunSpec.hs:340 opt-env-conf-test> ✗ 1 OptEnvConf.RunSpec.runParser.Unit tests.parses ["--foo","bar"] as ("--foo","bar") opt-env-conf-test> Retries: 3 (does not look flaky) opt-env-conf-test> Expected these values to be equal: opt-env-conf-test> Actual: ( "bar" , "--foo" ) opt-env-conf-test> Expected: ( "--foo" , "bar" ) ```