Upgrading markdown

The current markdown has some shortcomings and does not support a much wider range of markdown syntax (such as Github Flavored Markdown's fenced code blocks, instead of intending code blocks 4 spaces). Therefore, I've upgraded it to [Markdown Extra Extended](https://github.com/egil/php-markdown-extra-extended/), but with the current directory hierarchy, it didn't work properly. So I moved all the contents in the `vendor/` directory to `lib/` (with some required changes in the `index.php` at the top of the repo). You have now:

- The core of Pico in one single place.
- Use the **awesome** GFM and additional features such as figure and figcaption tags
- Excerpts are also parsed in markdown, previously all HTML tags were stripped, you probably don't want that since you might have code snippets, span's and so in your excerpt and stripping tags away would led to ugly excerpts

Also: some empty and thus useless `index.html` in some folders are removed (the CMS still works though)