arg was supported, just not passed along
@ -701,6 +701,7 @@ class InstallNBExtensionApp(BaseNBExtensionApp):
install = install_nbextension_python if self.python else install_nbextension
full_dests = install(self.extra_args[0],
destination=self.destination,
overwrite=self.overwrite,
symlink=self.symlink,
user=self.user,