feat: change markdown standard to GFM

Change markdown standard to Github Flavored Markdown (GFM) replacing Pandoc's
default markdown. This is consistent with the Jupyter Notebook markdown cells
and should improve compatibility.

doc/markdown-cells.ipynb

@@ -23,16 +23,47 @@
     "* Green\n",
     "* Blue\n",
     "\n",
-    "Note: JupyterLab and JupyterNotebook uses a different Markdown parser than nbsphinx (which currently uses Pandoc). \n",
-    "In case that your Bulletpoints do render in the notebook and do not render with nbsphinx, please add one blank line before the bulletpoints.\n",
-    "***\n",
+    "JupyterLab and JupyterNotebook use a different Markdown parser than nbsphinx\n",
+    "(which currently uses Pandoc). When using Pandoc v2.0 or later, nbsphinx uses\n",
+    "GitHub-flavored markdown, this is the same standard of markdown used by\n",
+    "JupyterLab and JupyterNotebook.\n",
     "\n",
+    "In particular, newer versions of nbsphinx (>v0.9.5) will now correctly convert\n",
+    "lists without a blank line:\n",
+    "\n",
+    "```markdown\n",
+    "No new line before this list:\n",
+    "1. One\n",
+    "2. Two\n",
+    "3. Three\n",
+    "```\n",
+    "No new line before this list:\n",
     "1. One\n",
-    "1. Two\n",
-    "1. Three\n",
+    "2. Two\n",
+    "3. Three\n",
+    "\n",
+    "\n",
+    "Arbitrary Unicode characters should be supported, e.g. łßō. Note, however, that\n",
+    "this only works if your HTML browser and your LaTeX processor provide the\n",
+    "appropriate fonts."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Task Lists\n",
     "\n",
-    "Arbitrary Unicode characters should be supported, e.g. łßō.\n",
-    "Note, however, that this only works if your HTML browser and your LaTeX processor provide the appropriate fonts."
+    "nbsphinx supports GitHub-style task lists:\n",
+    "\n",
+    "```\n",
+    "- [x] Task 1\n",
+    "- [ ] Task 2\n",
+    "- [ ] Task 3\n",
+    "```\n",
+    "- [x] Task 1\n",
+    "- [ ] Task 2\n",
+    "- [ ] Task 3"
    ]
   },
   {

src/nbsphinx/__init__.py

@@ -1039,13 +1039,22 @@ def filter_func(text):
         json_data = json.loads(text, object_hook=object_hook)
         return json.dumps(json_data)
 
-    input_format = 'markdown'
-    input_format += '-implicit_figures'
     v = nbconvert.utils.pandoc.get_pandoc_version()
-    if nbconvert.utils.version.check_version(v, '1.13'):
-        input_format += '-native_divs+raw_html'
     if nbconvert.utils.version.check_version(v, '2.0'):
-        input_format += '-smart'  # Smart quotes etc. are handled by Sphinx
+        """Accodring to https://nbformat.readthedocs.io/en/latest/format_description.html#markdown-cells
+        the kind of markdown used by jupyter is github flavored markdown (gfm).
+
+        The gfm extension was added in 2.0
+        (https://pandoc.org/releases.html#pandoc-2.0-2017-10-29).
+        """
+        input_format = 'gfm-smart'
+    else:
+        input_format = 'markdown'
+        input_format += '-implicit_figures'
+        if nbconvert.utils.version.check_version(v, '1.13'):
+            input_format += '-native_divs+raw_html'
+        if nbconvert.utils.version.check_version(v, '2.0'):
+            input_format += '-smart'  # Smart quotes etc. are handled by Sphinx
 
     rststring = pandoc(text, input_format, 'rst', filter_func=filter_func)
     rststring = re.sub(