Update configuration docs for MD extensions

Per `.mkdown` extension addition in my older pull request.
Update Site history based on History.markdown
Merge pull request #2102 from ZDroid/mkdown
Update history to reflect merge of #2102 [ci skip]
Update plugins.md

Added my plugin to the list.
Merge pull request #2105 from callmeed/patch-1
Update history to reflect merge of #2105 [ci skip]
Sass support should be shipped outside core.
Choose jekyll-sass-converter 1.0.0.rc1
Bring in jekyll-sass-converter
Merge pull request #2098 from jekyll/sass-as-separate-gem
Update history to reflect merge of #2098 [ci skip]
Clean out Sass tests that we don't need anymore.

Ref: #2098