require newline after start of yaml header
add test for PGP key yaml frontmatter
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Add an --unpublished option to render unpublished posts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
Add missing Command tests
Merge pull request #2216 from szymzet/command-better-coverage
Update history to reflect merge of #2216 [ci skip]
use regex to verify yaml existence
Stricter start of line check via \A
more explicit test description
Merge pull request #2211 from jekyll/pgp-key-header-frontmatter
Update history to reflect merge of #2211 [ci skip]
We're not allowing custom data sources anymore.
Merge pull request #1492 from maul-esel/publishing
Update history to reflect merge of #1492 [ci skip]