markdown: Add AutoLinking feature.

* @npcode
* SHA: 2022d330c5858eae9ca9cb5acb9e6a5060563b2c
* User/Project@SHA: me/foo@33f35dcd1d617918b9e5cda7a5bf0159a7ca1521
* #Num: #1
* User/Project#Num: me/foo#1