Add option::is_flag method

This new method is used for determining if an option is nullary. The
parser has been changed to use this method rather than manual checks.
Add range::allow_multiple method

This new method is used for determining if a range allowed multiple
instances.
Add formatting helpers

Add format documentation group
Add detail documentation group
Add clop::detail namespace

These new functions will be used in the formatting of automatically
generated interface documentation.
Add detail::word_wrap structure

This new structure will be used in the formatting of automatically
generated interface documentation.
Add usage structure

This new structure acts as an output manipulator for generating usage
information for an interface.