Pandoc requires no text before H1

[rgaiacs]

$ pandoc --from markdown --to markdown -s --shift-heading-level-by -1 <<EOF
# Title
EOF

produces

---
title: Title
---

This is the behaviour that we are looking. Unfortunatelly,

$ pandoc --from markdown --to markdown -s --shift-heading-level-by -1 <<EOF
foo

# Title
EOF

produces

foo

Title

where the title was transformed into a paragraph. The same happens when we have a comment:

$ pandoc --from markdown --to markdown -s --shift-heading-level-by -1 <<EOF
<!-- foo -->

# Title
EOF

produces

```{=html}
<!-- foo -->

Title