Support Markdown fenced code block inception

[ccoVeille]

Here is a an example

This

```console
$ pwd
/tmp
```

is rendered like this

$ pwd
/tmp

But to render the first block like this

```console
$ pwd
/tmp
```

I had to write this

````markdown
```console
$ pwd
/tmp
```
````

This format is used frequently in documentation to explain how to document things.

I would like at least that block starting with more than 3 backticks are interpreted the right way

So at least this should be detected by mdsf

````console
$ pwd     |      fgrep     tmp
/tmp
````

It's used frequently

https://github.com/search?q=%22%60%60%60%60shell%22+language%3AMarkdown+&type=code

The inception of formatting what is quoted inside another one might be not fun to achieve.