Add Markdown Post/Preprocessors

[caendesilva]

Key rule when adding shortcodes. They should be semantic. You shouldn't have to look up the syntax, instead the shortcode should be in a way that the source Markdown makes sense and is readable even when it is not being processed by Hyde.

For example, >info to make an info blockquote, in my opinion, makes immediate sense and lets met know that the string is meant to convey some info. As I have experience with Markdown, I also know that it will be rendered as a blockquote. And when I'm trying to remember how to make a blockquote that contains a warning, it's easy to remember that all I need to do is append the word warning to a normal blockquote.

Same with the new filepath shortcode, it transforms a syntax already commonly used -- like magic!

```php
// Filepath: hello.php

echo "A filepath label will automatically be added";
```

[caendesilva]

The system I'm working on now is pretty cool, but adds a lot of complexity. We'll see how it goes, maybe it belongs in a dedicated package.