Add a simple example for optparse documentation.

Also add a div that disallows syntax highlighting as a hack to
circumvent Doxygen's strange limitation. The hack only works on
IE11+. It should work on all other browsers.
Fix the help message for subcommands.
Add help and usage members for subcommand.

Improve overall formatting API for the subcommands. Keep the
error string as a member of the error exception.
Fix the help_formatter slicing issue.
Handle different types of errors in the parse member function
Add tutorial for optparse module.