Added support for passing PHPMarkdown parser configuration parameters

Patch allows the inclusion of parser parameters in the ```_content/config.yml``` file inside the ```markdown\config``` section, for example:

~~~ yaml
markdown:
    use_markdown_extra: true
    config:
        code_attr_on_pre: true
        fn_link_class: 'myfootnote'
~~~

The configuration above will set class parameters for fenced codes in the ```<pre>``` tag, instead of putting that in ```<code>```, and will also set ```myfootnote``` as the class used for the footnote links.