When converting Rst to markdown, and writing a fenced code block,
pandoc insert a space between the triple quotes and the options:

mistral-/tmp>cat bla.rst 
.. code:: python

    1+1

mistral-/tmp>pandoc bla.rst -t markdown
``` {.sourceCode .python}
1+1
```

However, when converting this further to a notebook, notedown seems
to get confused by this additional space, and does not produce a block
as desired.

Sorry, that later part is not super specific. I'll provide a complete
example later on, but right now I broke notedown on my machine while
hacking around ...

Would it be possible to tweak notedown's regexp to tolerate this
additional space?

Btw: thanks a lot for notedown! That's going to be super useful in my
Rst -> notebook workflow!