Add support for setting a function to handle flag parsing errors.

The default pflag error is to only print the bad flag. This enables an application
to include a usage message or other details about the error.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

doc/md_docs.md

@@ -101,4 +101,3 @@ linkHandler := func(name string) string {
 	return "/commands/" + strings.ToLower(base) + "/"
 }
 ```
-