feat(toc): add --depth option to limit heading level

[ekalinin]

Summary

Fixes #25: a way to keep only the top heading levels in the TOC.

--depth <NUM> keeps only headings with level <= NUM. The level is absolute, as
--depth in github-markdown-toc.go: --depth 2 keeps # and ##. An empty value or
0 means no limit, which is the current behavior.

• gh-md-toc:
  • gh_toc_grab: one condition at the start of the awk script shared by the default
    and OS/390 branches, so it applies to stdin, local files, remote URLs, several
    inputs and --insert. The value is passed with awk -v and compared as a number.
  • gh_toc_app: --depth is parsed right after --indent and before -, in the
    existing fixed order of options. gh_toc gets it as a new last parameter.
  • --help: a new line right after --indent.
• README.md: an example with --depth 1 in "Local files", without a new heading,
  so the README TOC asserted in the tests does not change.
• tests/tests.bats: tests for a local file (--depth 2) and stdin (--depth 1 -)
  with a new fixture tests/test directory/test_depth.md; test_help updated.
• openspec/changes/add-toc-depth: proposal, design, spec and tasks.

Not included: --start-depth, the --depth=NUM syntax, validation of the value,
changes to indentation (#30, #75), options in any order.

Testing

• The new tests fail without the change; all 17 tests pass with it.
• --depth 0 gives the same output as without the option.
• Checked manually: a remote URL with --depth 1 and --insert with --depth 1.
• make lint passes.