Add support for internal or hidden flags

[GoogleCodeExporter]

There are times where a user might want to define a flag for internal use,
but not provide it in the help output.

Original issue reported on code.google.com by [email protected] on 1 Apr 2009 at 2:19

[GoogleCodeExporter]

Original comment by [email protected] on 6 Jan 2013 at 4:38

• Changed title: Add support for internal or hidden flags
• Added labels: Priority-Low
• Removed labels: Priority-Medium