Add inspect filter

[chibicode]

For #2814 - I opted out for debug instead of inspect to mimic the debug helper method in Rails (their implementation). It also escapes HTML tags.

Usage:

<pre>{{ page | debug }}</pre>

Output:

<pre>

---
layout: default
content: |
  ...
name: index.html
path: index.html
url: &quot;/&quot;

</pre>

[chibicode]

The test seems to be failing on something irrelevant https://travis-ci.org/jekyll/jekyll/builds/33869200

[parkr]

This is pretty cool. I still like inspect better because it's more in line with Ruby (most of our users aren't Rails developers anyway), but the code looks ok. Thanks!

[chibicode]

@parkr Thanks, renamed!

[parkr]

Why not input.inspect?

[chibicode]

Again I was basing off Rails's debug and thought that to_yaml would have richer information. But if inspect suffices, will change to it.

[parkr]

Also if you rebase on the latest master, you'll find that the tests should pass. 👍

[chibicode]

@parkr Thanks, cleaned it up at #2867!