Plugin reference: https://jekyllrb.com/docs/plugins/ & https://pages.github.com/versions/ → https://github.com/benbalter/jekyll-optional-front-matter → https://github.com/olov/jekyll-references → https://github.com/amedia/premonition → https://github.com/kinnetica/jekyll-plugins → https://help.github.com/articles/search-engine-optimization-for-github-pages/