WIP: DDH longtail summaries

Here's the current plan:
Summaries are added inside an HTML comment.
Add the title in an H2 tag with a `summary` class
Anything you want to show up in the longtail document is wrapped in a span tag with `summary-text` class.
The text can be written in HTML, markdown, or a mix of both.  If using mardown add `markdown="1"` class to the span.  


@jdorweiler is there any chance we could use our own tags here (XML) rather that valid HTML tags with classes? I'm thinking it would be quicker/easier/cleaner if we could have:

```html
<!--
<summary>
    <moreat>goodie_basic_tutorial</moreat>
    <title>sfsdfsfsfsfs</title>
    <markdown>this is some markup **text**</markdown>
</summary>
-->
```