Fixes #33: Replaced the Markdown parser

Implements #33 

- replaced the Markdown parser to Parsedown Extra
- support for CommonMark (i.e. main thing is fenced code blocks like on GitHub)
- replaced `[include url="<url>" escape="true/false"]` with `{@include <url>}` and `{@include escape <url>}` for simplicity and consistency with the custom tags that we can use in PHP comments, and also I think that it has less chances of conflicts with Markdown syntax (the old syntax reused link syntax which could be confusing)
- refactored the Markdown parsing and custom additions (using an interface and the decorator pattern)
- added explicit language for some codeblocks (especially the remote-included ones) for more reliable syntax highlighting: possible thanks to fenced code blocks
- replaced some inline HTML `<pre><code></code></pre>` with fenced code blocks (I think it was previously necessary because impossible to set an indentation on remote-included files)