Strip paragraph attributes inside fenced divs during GFM export

[RensDimmendaal]

Fix md2gfm leaving paragraph attributes in its output when the paragraph is inside a fenced div.

Octavo uses this markup to style blog cards, e.g.:

::: {.listing-card data-tags="|technical|"}
## [Git Friendly](</blog/posts/2022-08-25-git/index.html>)

Aug 25, 2022 · Jeremy Howard
{: .listing-meta}  # <-- these were left in on main

Solving [git] conflicts with `git` | nbdev
{: .listing-description}

[technical](<#tag=technical>){.listing-topic}
{: .listing-topics}  # <-- these were left in on main

![](</blog/posts/2022-08-25-git/cover.png>)
:::

w this PR it turns into:

## [The Jupyter+git problem is now solved](<posts/2022-08-25-jupyter-git/index.md>)

Aug 25, 2022 · Jeremy Howard

Previously, using git with Jupyter could create conflicts and break notebooks. With nbdev2, the problem has been totally solved.

[technical](<#tag=technical>)

![](<posts/2022-08-25-jupyter-git/unreadable-notebook.png>)