autolink project modules and function in readme file fixes #158

* 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="">'
Merge pull request #159 from tonini/autolink-readme

autolink project modules and functions in readme file
Release v0.6.1