Fixes #9.
Merge branch 'issue-9' into develop
Fixes #10.
Merge branch 'issue-10' into develop
Allow arguments to be required.  Fixes #11.
Merge branch 'issue-11' into develop
Fixes #13.
Merge branch 'issue-13' into develop
Better help, especially showing defaults.  Fixes #12.
Merge branch 'issue-12' into develop
Prefix io filter.
Merge branch 'issue-4' into develop
Added abort filter.
Merge branch 'issue-15' into develop
Added LineReader class.
Merge branch 'issue-16' into develop
Improve default printing in --help, and add an option to Args to not print defaults.
Merge branch 'issue-12' into develop
Fix compilation issue.
Merge branch 'issue-12' into develop