Expose page.unrendered_content and page.rendered_content to Liquid

## Problem

In Liquid template, `page.content` variable represents both un-rendered content and rendered content (e.g. un-rendered content = `[Jekyll](http://jekyllrb.com)`, rendered content = `<p><a href="http://jekyllrb.com">Jekyll</a></p>`).

This behaviour is documented [as follows](http://jekyllrb.com/docs/variables/): 

    The content of the Page, rendered or un-rendered depending upon what Liquid is being processed and what page is.

By further code reading, I found how to refer to (un)rendered content.

                           | un-rendered content| rendered content
---------------------------|--------------------|--------------------------------------
un-rendered page           | `page.content`     | impossible
rendered & layouting page  | `page.content`     | `content` (If the page is already wrapped by other layout file, `content` isn't equal to rendered content)
rendered & layouted page   | ----               | `page.content`

It would be useful if we could get (un)rendered content anytime.

For example, I want to embed `<og:description>` tag to my new site generated by `jekyll new` command.

* Adding `<meta property="og:description" content="{{ content | strip_newlines | strip_html | escape}}">` to `_include/head.html` isn't good because `og:description` contains site name and post date.
* add `<meta property="og:description" content="{{ page.content | strip_newlines | strip_html | escape}}">` to `_include/head.html` isn't good because `og:description` contains raw markdown document.

I need a new Liquid variable to embed rendered content.


## Solution

I exposed `page.unrenderd_content` and `page.rendered_content` variable to Liquid. Now, it becomes obvious how to use (un)rendered content.

                           | un-rendered content    | rendered content
---------------------------|------------------------|----------------------------------
un-rendered page           | page.unrenderd_content | page.rendered_content (== nil)
rendered & layouting page  | page.unrenderd_content | page.rendered_content
rendered & layouted page   | page.unrenderd_content | page.rendered_content

The behavior of `page.content` isn't changed in order to keep the backward compatibility.