add fenced_code extensions
add fenced_code extensions

This markdown extension support code block with tilde (```) syntax like github
mgaitan/waliki_flask,alexanderjulo/wiki,alexex/wiki,JoeyBoolean/wiki,undergroundtheater/wiki,mgaitan/waliki_flask,Tony1928/flask-wiki,JoeyBoolean/wiki,Tony1928/flask-wiki,NolanFoster/wiki,alexanderjulo/wiki,NolanFoster/wiki