Render and sanitize readme.md just like github would.
Basically, it's node.js version for this: https://github.com/github/markup
But only for markdown for now, because that's what io.js packages use mostly.
'Hello, *world*!'// outputs '<p>Hello, <em>world</em>!</p>\n'
<a name="blah">instead of
<a id="user-content-blah">, but should be good enough)