autolink project modules and function in readme file

@josevalim could you give me some feedback about this?

I'm not sure about the changes in `Autolink`, but now `project_doc` can be used without any kind of preparation of getting functions/modules name lists before calling it. Because there is just the info about the modules in the `HTML` module, I thought that would be the proper way to use. 

* refine `Autolink` module and `Autolink.project_doc/1` function to use it properly outsite of the Module
* `HTML.write_readme/4` uses `Autolink.project_doc/1` now to autolink readme file content
* fix 'codeblock' string replacement in `HTML.write_readme/4`. plain used \`\`\` blocks are parsed as `<pre><code class="">`