Adding Markdown to Ddoc

[Chris]

Compare:

Output:
https://github.com/vibe-d/vibe.d/blob/master/README.md

Input:
https://raw.githubusercontent.com/vibe-d/vibe.d/master/README.md

Most programmers who use GitHub will be familiar with this and 
can write docs, tutorials etc. with very little effort. The set 
of Markdown used for the D documentation shouldn't be needlessly 
limited (i.e. it shouldn't offer less than CommonMark - GitHub's 
core markup langauge).