tag: add passthrough for positional arguments

This new tag tells the parser to stop processing flags after the
positional argument is encountered.

This is particularly useful for subcommands that forward their arguments
to an external program, and makes it possible to implement commands
like `kubectl exec` or `docker run`.

Fixes #80.

build.go

@@ -195,6 +195,7 @@ func buildField(k *Kong, node *Node, v reflect.Value, ft reflect.StructField, fv
 		Tag:          tag,
 		Target:       fv,
 		Enum:         tag.Enum,
+		Passthrough:  tag.Passthrough,
 
 		// Flags are optional by default, and args are required by default.
 		Required: (!tag.Arg && tag.Required) || (tag.Arg && !tag.Optional),