Tweaking slug logic in post settings.

Closes #2136.
- sets slug to placeholder text if slug is blank
Merge pull request #2151 from halfdan/2123-tags-suffix

Allow HTML inside tag prefix/suffix
Merge pull request #2147 from gmurphey/master

Tweaking slug logic in post settings.
Update express-hbs to fix block helper issue.

fixes #2127
- Update express-hbs to 0.7.9