Add debug filter

For https://github.com/jekyll/jekyll/issues/2814 - I opted out for `debug` instead of `inspect` to mimic the [`debug` helper method in Rails](http://guides.rubyonrails.org/debugging_rails_applications.html#debug). It also escapes HTML tags.

## Usage:

```html
<pre>{{ page | debug }}</pre>
```

#### Output:

```html
<pre>
---
layout: default
content: |
  ...
name: index.html
path: index.html
url: &quot;/&quot;

</pre>
```