Refactored npmpackagejson to enable multiple files

[ghost]

Hello, thank you for this really handy tool!

I use it in a monorepo PWA, which contains several package.json files in subfolders. To track them all, I refactored the npmpackagejson plugin. I also updated the PLUGINS.md to reflect the changes. The previous behavior is not altered.

Please find here the test file.
To make it work:

• rename the file test.sh (Github does not allow the uploading of .sh files)
• install git
• install Node.js (with default package manager npm)
• create an empty folder
• populate this folder with:
  • test.sh
  • .git-semver/
    |-> plugins/
    |-> npmpackagejson.sh
• run chmod 766 test.sh && ./test.sh

Please let me know if there is anything I can do to enhance this PR!