Update configuration.md

repeat warning about `destination` that is found in <http://jekyllrb.com/docs/usage/>
(see also pull request #3159)
Add jekyll-500px-embed to tag plugins
Merge pull request #3161 from dnozay/patch-3
Update history to reflect merge of #3161 [ci skip]
Merge pull request #3163 from lkorth/patch-1
Update history to reflect merge of #3163 [ci skip]
Allow documents to set `published` in front matter
Expose Publisher in Site