[Extension] showdown-spoiler, support for spoiler tags within markdown

This extension lets you use double pipes to hide spoilers. They can be revealed by clicking on the hidden section. It has no effect inside of `<pre>` and `<code>` sections.

```
This is a ||spoiler||.

It also can ||spread  
across line breaks||
```

Link to repo: https://github.com/kvmet/showdown-spoiler