feat: support multi inputs

[kenji-miyake]

Resolves #78

This is an improved version of #178.
Although the diff is large, since I split commits into small units, I suppose you can understand what I changed by seeing each commit.

How to test

$ git clone git@github.com:kenji-miyake/markdown-link-check.git -b support-multi-inputs
$ cd markdown-link-check
$ git cherry-pick 7b3ad2c1ce5e49d03ff29cbebd6951006b0ccf4a
$ # pre-commit
$ pre-commit clean
$ pre-commit run -a
[WARNING] The 'rev' field of repo '.' appears to be a mutable reference (moving tag / branch).  Mutable references are never updated after first install and are not supported.  See https://pre-commit.com/#using-the-latest-version-for-a-repository for more details.  Hint: `pre-commit autoupdate` often fixes this.
[INFO] Initializing environment for ..
[INFO] Installing environment for ..
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
Markdown Link Check......................................................Failed
- hook id: markdown-link-check
- exit code: 1


FILE: test-2.md

  1 links checked.

FILE: test-1.md

  1 links checked.

FILE: test-3.md
  [✖] http://common-invalid-link.example.com
  [✖] http://invalid-link3.example.com

  3 links checked.

  ERROR: 2 dead links found!
  [✖] http://common-invalid-link.example.com → Status: 0
  [✖] http://invalid-link3.example.com → Status: 0

FILE: test-4.md

  1 links checked.

$ # CLI
$ sudo npm install -g .
$ markdown-link-check test-1.md test-2.md test-3.md test-4.md

FILE: test-1.md
  [✓] http://google.com

  1 links checked.

FILE: test-2.md
  [✓] http://google.com

  1 links checked.

FILE: test-3.md
  [✓] http://google.com
  [✖] http://common-invalid-link.example.com
  [✖] http://invalid-link3.example.com

  3 links checked.

  ERROR: 2 dead links found!
  [✖] http://common-invalid-link.example.com → Status: 0
  [✖] http://invalid-link3.example.com → Status: 0

FILE: test-4.md
  [✓] http://google.com

  1 links checked.

[kenji-miyake]

@tcort Hello, thank you for this useful tool!
Could you take a look at this, please? 🙏

[kenji-miyake]

@tcort Hi, I know you are busy, but let me friendly ping.

[kenji-miyake]

@tcort Thank you so much! 😄

[waylan]

Shouldn't the documentation be updated to reflect this change (and demonstrate how to call with multiple files)?

markdown-link-check/README.md

Lines 144 to 153 in 458cefb

	#### Check links from a local markdown folder (recursive)
	Avoid using `find -exec` because it will swallow the error from each consecutive run.
	Instead, use `xargs`:
	```shell
	find . -name \*.md -print0 | xargs -0 -n1 markdown-link-check
	```
	There is an [open issue](https://github.com/tcort/markdown-link-check/issues/78) for allowing the tool to specify
	multiple files on the command line.

[matschaffer]

I had the same question as @waylan - is there a way to enable recursive check using a cli arg? I'm using the docker command to try this out for starters. --help on that doesn't seem to list any "recursive" option.