Gemroff v7

- Add new .FnO and .FnC line types for footnote blocks
- Parsing: substantially more robust parsing and error checking
- Configuring: Add new Images_Hidden config key to put HTML images in <details>
  block
- Rendering: Add generic Render function taking an Output_Format argument
- Rendering.*: Support nested footnotes properly (more than one level
  of nesting)
- Rendering.Render_HTML: less horrible opening <p> placement
- Rendering.Render_HTML: fix bug where errant spaces would be placed after <br/>
- Rendering.Render_HTML: implement footnote blocks
- Rendering.Render_HTML: fix bug where header fragments would not be downcased
- Rendering.Render_{Gemtext,Text,TTY}: Always include textual footnotes even if
  inline links are omitted
- Rendering.Render_{Text,TTY}: Also hard wrap rendered footnotes
- contrib: update contrib scripts with new line types
- docs: add known issue about stray newlines appended to files
- docs: update typos and add clarifications
- docs: add documentation for footnote blocks
- docs: add documentation for the new images-hidden config option
- docs: add known issue about stray newlines appended to files
- docs: regenerate markdown versions
- docs: manually reformat markdown conversions of man pages
- docs: completely hand-write summary in gemroff.5.md
- docs: remove outdated elements.md
- README: add additional contact info
- README: have build instructions checkout stable version
- README: add Gemroff.Utils to API docs listing
- tests: more footnote testing
- tests: add footnotes.gr integration test
- tests: add new integration test
- tests: have run_tests.sh load a specific config for each test
- tests: add custom config for prose.gr
- tests: have elements.gr test replace-extension
- .builds: update for new Debian stable
- .builds: add Fedora build script to supplement Debian