Add reStructuredText directive post_list for issue #815
Fix ralsinas comments.
Fix handling of empty tag option.
Remove unused import
Use slice on posts only, for more predictible results.
Add slugs option.
Add *all* option.
Fix handling of directive.flag. When flag is set, None is returned.
Merge pull request #816 from udono/master

Add reStructuredText directive post_list for issue #815