Support collapsable <details> blocks

[amomchilov]

In technical writing, it's common to want to include collapsible sections for content that might be useful, but which you don't want to spam out the full height of your post. E.g. a large code listing.

For comparison, GitHub supports this via the standard <details> tag, whereas Discourse has its own [details="Summary"] ... [/details] syntax.

Kramdown already tolerates at least some HTML input (e.g. <p>, <a>, <b>, <u>, etc.), including <details>. However, it doesn't process any of the MarkDown markup that might be inside that body, so e.g. it's impossible to use MarkDown links or code blocks within collapsible sections.

Example:

<details>
<summary>Click to expand</summary>

[Example link](http://example.com)

``` ruby
p "Hello, world!"
```

> quote block

</details>

GitHub rendering:

Click to expand

Example link

p "Hello, world!"

quote block