Set marked to use gfm rather than pedantic rules

We remove `pedantic`, since `marked` defaults to `gfm` being true.

lib/metalsmith.js

@@ -217,7 +217,6 @@ module.exports = metalsmith(__dirname) // __dirname defined by node.js: name of
 
       // Markdown engine options
       smartypants: true, // use "smart" typographic punctuation
-      pedantic: true,
       mangle: false,
       highlight: highlighter
     }