Feature/add support for multiple makefiles

[trinitronx]

Description

As a follow-up to #69, this PR adds support for passing multiple Makefile / *.mk paths as command-line arguments, just like how pre-commit run --all-files will execute checkmake when a repo contains multiple *.mk files.

Checklist

Not all of these might apply to your change but the more you are able to check the easier it will be to get your contribution merged.

• [x] CI passes
  • Tests passing on my fork here
• [x] Description of proposed change
  • See above
• [x] Documentation (README, docs/, man pages) is updated
  • See README.md, man/man1/checkmake.1.md file diffs
• [x] Existing issue is referenced if there is one
  • Follow-up to #69
  • Avoids usage errors when running checkmate via pre-commit run --all-files
• [x] Unit tests for the proposed change
  • See *_test.go file diffs

[mrtazz]

thanks for taking the time to contribute!