[ekalinin]

Summary

Fixes #71: a way to start TOC entries with a dash instead of an asterisk.

--bullet <CHAR> takes *, - or + (the three bullet markers of CommonMark) and
starts every bullet entry with it: - [Title](#title). The default stays *, so the
output without the option is the same as before.

• gh-md-toc:
  • gh_toc_grab: the bullet is passed to the awk script with awk -v and replaces
    * in the entry marker. The script is shared by the default and OS/390 branches,
    so it applies to stdin, local files, remote URLs, several inputs, --insert and
    --numbered outline (- [2.1. Section](#section)).
  • gh_toc_app: --bullet is parsed right after --numbered and before -, in the
    existing fixed order of options, so --bullet - - reads stdin with dash bullets.
    gh_toc gets it as a new last (10th) parameter, read as ${10}.
  • Errors with exit code 1, before any network call: an unknown character, and
    --bullet with --numbered list (an ordered list has no bullet).
  • --help: a new line right after --numbered.
• README.md: an example in "Local files" after the --numbered outline example,
  without a new heading, so the README TOC asserted in the tests does not change.
• tests/tests.bats: tests for a local file with -, stdin with --bullet - -, +
  with --numbered outline, and both errors, on the existing fixtures; test_help
  updated.
• openspec: the add-toc-bullet change (proposal, design, specs, tasks) is archived,
  with a new toc-bullet spec and "Hierarchical numbers" in toc-numbered updated for
  the bullet.

Not included: other bullet characters, a different bullet per level, a change of the
default, the --bullet=CHAR syntax.

Testing

• The new tests and the updated help tests fail without the change; all 34 tests pass
  with it.
• Checked manually: a remote URL with --bullet -, two local files with --bullet +,
  and --insert --no-backup with --bullet - on a copy of README.md.
• make lint passes.