Add `space_in_atx_header` extension.

This is enabled by default in pandoc and GitHub markdown but not the
other flavors.

This requirse a space between the opening #'s and the header
text in ATX headers (as CommonMark does but many other implementations
do not).  This is desirable to avoid falsely capturing things ilke

    #hashtag

or

    #5

Closes #3512.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/command/3512.md b/test/command/3512.md
new file mode 100644
index 000000000..cf8c72e58
--- /dev/null
+++ b/test/command/3512.md
@@ -0,0 +1,13 @@
+```
+% pandoc -f markdown-auto_identifiers
+#hi
+^D
+<p>#hi</p>
+```
+
+```
+% pandoc -f markdown-auto_identifiers-space_in_atx_header
+#hi
+^D
+<h1>hi</h1>
+```