Print global options in help for subcommands

fixes #101

example_test.go

@@ -235,7 +235,8 @@ func Example_helpTextForSubcommand() {
 	// Positional arguments:
 	//   ITEM                   item to fetch
 	//
-	// Options:
+	// Global options:
+	//   --verbose
 	//   --help, -h             display this help and exit
 }
 

usage.go

@@ -144,10 +144,30 @@ func (p *Parser) writeHelpForCommand(w io.Writer, cmd *command) {
 	}
 
 	// write the list of options
+	if len(options) > 0 || cmd.parent == nil {
 		fmt.Fprint(w, "\nOptions:\n")
 		for _, spec := range options {
 			p.printOption(w, spec)
 		}
+	}
+
+	// obtain a flattened list of options from all ancestors
+	var globals []*spec
+	ancestor := cmd.parent
+	for ancestor != nil {
+		for _, spec := range ancestor.specs {
+			globals = append(globals, spec)
+		}
+		ancestor = ancestor.parent
+	}
+
+	// write the list of global options
+	if len(globals) > 0 {
+		fmt.Fprint(w, "\nGlobal options:\n")
+		for _, spec := range globals {
+			p.printOption(w, spec)
+		}
+	}
 
 	// write the list of built in options
 	p.printOption(w, &spec{