Rename Standard Markdown to CommonMark

http://blog.codinghorror.com/standard-markdown-is-now-common-markdown/

https://github.com/coding-horror/coding-horror.github.io/commit/2d37920c39d15db0769f710bd2414e9a2ed7b565
Merge pull request #218 from hkdobrev/common-mark

Rename Standard Markdown to CommonMark