Add example for "global" options (#1708)

* Add example for common-options

* Rename example so easier to find

examples/global-options.js

@@ -0,0 +1,51 @@
+#!/usr/bin/env node
+
+// This example shows a couple of ways to add a "global" option to all of the subcommands.
+// The first approach is to use a subclass and add the option as the subcommand is created.
+// The second approach is to loop over the subcommands after they have been created.
+//
+// The code in this example assumes there is just one level of subcommands.
+//
+// (A different pattern for a "global" option is to add it to the root command, rather
+// than to the subcommand. That is not shown here.)
+
+// const { Command } = require('commander'); // (normal include)
+const { Command } = require('../'); // include commander in git clone of commander repo
+
+// Common options can be added when subcommands are created by using a custom subclass.
+// If the options are unsorted in the help, these will appear first.
+class MyRootCommand extends Command {
+  createCommand(name) {
+    const cmd = new Command(name);
+    cmd.option('-v, --verbose', 'use verbose logging');
+    return cmd;
+  }
+}
+
+const program = new MyRootCommand();
+
+program.command('print')
+  .option('--a4', 'Use A4 sized paper')
+  .action((options) => {
+    console.log('print options: %O', options);
+  });
+
+program.command('serve')
+  .option('-p, --port <number>', 'port number for server')
+  .action((options) => {
+    console.log('serve options: %O', options);
+  });
+
+// Common options can be added manually after setting up program and subcommands.
+// If the options are unsorted in the help, these will appear last.
+program.commands.forEach((cmd) => {
+  cmd.option('-d, --debug');
+});
+
+program.parse();
+
+// Try the following:
+//    node common-options.js --help
+//    node common-options.js print --help
+//    node common-options.js serve --help
+//    node common-options.js serve --debug --verbose