Ignore lines in code blocks

[andreaferretti]

Right now, the plugin misunderstands comment lines inside code blocks for markdown headings. For instance, when writing

```python
# this is a comment
```

the this is a comment line is mistakenly considered a heading

[betrue12]

Said at #45 and I sent pull request #61 :smiley:

[hsxfjames]

also in css code blocks:

    ```css  
    #someClass { width:100%; }  
    ```

[gaelL]

Hello, I'm in 0.4.2 and still have the same issue

With a simple block code as

```
# foo
```

[FabianNiehaus]

Still present in 2020.