Start adding support for parser groups.
Stash changes.
Factor out _add_item().
Pull out _Section class.
Remove _root_section and _current_section attributes.
Remove the _Section.formatter attribute.
Add HelpFormatter.format_section_heading().
Remove _Section.parent.
Remove _Section.format_help().
Combine some message-building methods.
Add description to _SectionNode.
Move implementation of parser.format_help() to Formatter.
Move format_usage() logic to Formatter.
Start refactoring self._current_indent away.
Remove _current_indent attribute.
Simplify format_help().
Rename Formatter.add_argument().
Work on traverser.
Compute max action length using traversal.
Change implementation of _add_item().