only works for markdown source, not HTML

[neon-ninja]

Simple repro:

pandoc -f markdown -t docx -o test.docx --filter=pandoc-docx-pagebreakpy README.md

Outputs Page Break

pandoc -f markdown -t html -o - --filter=pandoc-docx-pagebreakpy README.md | pandoc -f html -t docx -o test.docx --filter=pandoc-docx-pagebreakpy

No output

[neon-ninja]

Here's a fix #4