Fixes #33: Replaced the Markdown parser

[mnapoli]

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)

[mattab]

More consistency and features, looks nice! Feel free to merge it when it suits

[mattab]

Noticed some regressions @mnapoli

• on metadata the following _URL = index.php?module=API&method=ImageGraph.get&idSite=7&apiModule=UserSettings&... but it should be italic instead.
• on segmentation there is
  &segment=country==IN Return results where the country is India
  but the backtick should have transformed to code ?

[mnapoli]

@mattab fixed it, those guides were using really weird syntaxes and HTML elements that don't even exist anymore in HTML5 ;)

[mnapoli]

btw we can now use tables instead of HTML in Markdown files: d817a52 (much easier to read)