Better argument parsing

[ixs]

Currently gh-md-toc will only do a limited job parsing arguments.

e.g. gh-md-toc --insert --hide-footer --skip-header README.md would fail as only the --insert option would be parsed as an option and the --hide-footer option would be considered the filename.

This PR adds a new parser in a loop that will parse all arguments instead.

The parser is inspired by https://stackoverflow.com/a/14203146, which solves the problem nicely.

[ekalinin]

Thanks for the PR. A few things need to be fixed before it can be merged:

1. Conflicts with master. The PR is based on 0.9.0, and master has since added --depth and --numbered. The new loop has no cases for them, so they fall into the -* branch and print help. The --numbered checks (type, and --indent 3 or more for list) also need to run after the loop. Please rebase and add both options. The tests for #25 and #26 cover them.

2. --indent as the last argument hangs. gh-md-toc README.md --indent never exits: shift 2 fails without shifting, so the loop spins forever. Please check that the value is present.

3. Unknown options print help and exit 0. gh-md-toc --insert --indnet 2 README.md exits 0, so a typo in CI passes silently while the TOC is never updated. It should print an error and exit non-zero.

4. - is handled in the middle of parsing. The -) branch returns right away, so options after - are ignored, files before - are dropped, and --insert / --skip-header before - do nothing. - should be collected as a source like any file and processed after the loop.

5. make lint fails. The rewritten block brings back warnings fixed on master in 1c9e38a (SC2166, SC2006/SC2046, SC2155, SC2086, SC2162) and adds SC2317 and SC2221/SC2222. Keeping master's version of the stdin code after the rebase fixes most of them.