Support multi inputs (yet a prototype)

[kenji-miyake]

Overview

Resolves #78

Supported multi inputs like markdown-link-check file1.md file2.md.

Note: As I'm almost a newbie of Javascript, the code is not so good, but I guess it worked. I'll appreciate it if somebody would improve my code.

How to test

With This PR (pre-commit)

$ git clone git@github.com:kenji-miyake/markdown-link-check.git -b support-multi-inputs-prototype
$ cd markdown-link-check
$ 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.
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.

With This PR (CLI)

# Run git clone as above
$ sudo npm install -g . # Or something like that to install the local package
$ 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.

Without This PR (pre-commit)

$ git clone git@github.com:kenji-miyake/markdown-link-check.git -b support-multi-inputs-prototype
$ cd markdown-link-check
$ git checkout 7b3ad2c1ce5e49d03ff29cbebd6951006b0ccf4a
$ 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......................................................Passed

Without This PR (CLI)

# Run git clone as above
$ markdown-link-check test-1.md test-2.md test-3.md test-4.md && echo "Passed"

FILE: test-1.md
[✓] http://google.com

1 links checked.
Passed

[kenji-miyake]

Refactored in #179.