im(arg_enum): allows ascii case insensitivity for enum variants
im(arg_enum): allows ascii case insensitivity for enum variants

Allows creating an enum with CamelCase to follow Rust guidelines, but
then will match ascii case insensitive. This means variant
SomeEnum::Emacs would match string "emacs".

Closes #104

sru/clap-rs,wezm/clap-rs,kbknapp/clap-rs,Vinatorul/clap-rs,darayus/clap-rs,clap-rs/clap,kbknapp/clap-rs,sru/clap-rs,nelsonjchen/clap-rs,darayus/clap-rs,daboross/clap-rs,clap-rs/clap,clap-rs/clap,daboross/clap-rs,wezm/clap-rs,wezm/clap-rs,nelsonjchen/clap-rs,daboross/clap-rs,Vinatorul/clap-rs,cstorey/clap-rs,sru/clap-rs,grossws/clap-rs,grossws/clap-rs,cstorey/clap-rs,Vinatorul/clap-rs,cstorey/clap-rs,grossws/clap-rs,darayus/clap-rs,nelsonjchen/clap-rs