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.

tag.go

@@ -34,6 +34,7 @@ type Tag struct {
 	Embed       bool
 	Aliases     []string
 	Negatable   bool
+	Passthrough bool
 
 	// Storage for all tag keys for arbitrary lookups.
 	items map[string][]string
@@ -184,6 +185,11 @@ func parseTag(fv reflect.Value, ft reflect.StructField) *Tag {
 		t.PlaceHolder = strings.ToUpper(dashedString(fv.Type().Name()))
 	}
 	t.Enum = t.Get("enum")
+	passthrough := t.Has("passthrough")
+	if passthrough && !t.Arg {
+		fail("passthrough only makes sense for positional arguments")
+	}
+	t.Passthrough = passthrough
 	return t
 }