Gemroff v9

- Gemroff.Parsing: Parse paragraph breaks in gemtext mode
- Makefile: Properly clean up potentially remnant test artifacts
- Config: Allow setting replace-extension to remove .gr extensions outright
- gemroff.conf.5: update replace-extension documentation to be accurate
- Gemroff.Configuring: Add distinct 'replace-extension' and
  'extension-replacement' keys
- gemroff.conf.sample: default to commenting out the default sample lines
- Gemroff.Configuring: fix bug from old replace-extensions code
- Tests: update tests for new extension replacement
- Gemroff.Configuring: much more clear error message when configuration file
  doesn't exist
- Gemroff.Rendering.Render_HTML: wrap <img> in <a> to allow clicking image to
  open full-size
- Gemroff.Parsing: Fix bug in gemtext mode when encountering preformatted alt
  text without space between it and the ```
- contrib/is_gemtext_gemroff.awk: check for incompatibilities relating to
  preformatted alt text
- Gemroff.Rendering.Render_HTML: add value attribute to make footnotes have
  proper numbers