Feature: Support Github Flavor notes

In Github flavored markdown, a note or "alert" can be created with the following syntax:

```
> [!Note]
> *insert note here*
```

and the note will appear with an icon like so:

> [!Note]
> this is a note

Here is an screenshot:


`[!Important]` and `[!Warning]` have a similar layout.

This is not supported in markdown preview, and I would love to see it implemented. I understand, however, if the goal of the plugin is to support vanilla markdown, not the GitHub flavor.