Add `.mkdown` as valid Markdown extension

GitHub Linguist recognizes `.mkdown` as Markdown, so Jekyll should support it as Jekyll is used on GitHub pages.
Reorder Maruku and Kramdown

Kramdown is default now, yay!
Merge pull request #2049 from ZDroid/kramdown-reorder
Update history to reflect merge of #2049 [ci skip]
Merge pull request #2048 from ZDroid/mkdown-ext
Update history to reflect merge of #2048 [ci skip]