fix: make --quiet flag only output errors

[tcrasset]

Closes issue #223

Before

$ ./markdown-link-check test/* --quiet

FILE: test/file.md

FILE: test/hash-links.md
  [✖] #potato
  [✖] #tomato

  4 links checked.

  ERROR: 2 dead links found!
  [✖] #potato → Status: 404
  [✖] #tomato → Status: 404

FILE: test/hello.jpg

  0 links checked.

FILE: test/local-file.md

FILE: test/markdown-link-check.test.js

  0 links checked.

FILE: test/sample.md

FILE: test/special-replacements.md

After this change

$ ./markdown-link-check test/* --quiet

  ERROR: 2 dead links found in test/hash-links.md!
  [✖] #potato → Status: 404
  [✖] #tomato → Status: 404

On a file without broken links, it doesn't print anything:

Before

$ ./markdown-link-check test/file.md --quiet

FILE: test/file.md

After

$ ./markdown-link-check test/file.md --quiet
$