added ability to configure the Markdown parser

I really like Phrozn, but wanted to make it easier to use [Google Prettify](https://code.google.com/p/google-code-prettify/) within Markdown. It's possible to configure the Markdown parser to include the required 'prettyprint' class on the `<pre>` tag, but there was no way to do that within Phrozn.

This pull request corrects that. It's a fairly simple change only to the Markdown processor to look for likely configuration settings in both the front matter and the sitewide config.yml. While this opens up all the [configurable parts of Markdown Extra](https://michelf.ca/projects/php-markdown/configuration/), my original goal of making code blocks play with Google Prettify is achieved by adding this to config.yml

    markdown:
        code_class_prefix: "prettyprint "
        code_attr_on_pre: true