document commonmarker

it seems jekyll-commonmark-ghpages is relying on commonmarker:

jekyll-commonmark-ghpages/lib/jekyll-commonmark-ghpages.rb

Line 4 in b49878c

require "commonmarker"

however i dont see that this is documented anywhere

this is concerning because jekyll-commonmark-ghpages options are really
commonmarker options, so someone seeking the options might be confused as they
would need to visit commonmarker page to see the full list:

http://github.com/gjtorikian/commonmarker