fix(AllowLeadingHyphen): fixes an issue where  isn't ignored like it should be with this setting
fix(AllowLeadingHyphen): fixes an issue where  isn't ignored like it should be with this setting

Prior to this fix, using `AppSettings::AllowLeadingHyphen` wouldn't properly ignore `--`

Closes #558

kbknapp/clap-rs,clap-rs/clap,clap-rs/clap,clap-rs/clap,kbknapp/clap-rs