add a cucumber feature to test "published" for pages
Add an --unpublished option to render unpublished posts

Fixes #1337, #469.
add the Publisher class to handle publishing logic
Use Publisher instead of Post#published?
Support "published: false" for pages

Fixes 1034.
Add feature for the "--unsupported" flag

Also make step definitions a little more generic.
delete old superfluous tests; extend feature
Merge pull request #1492 from maul-esel/publishing