Improve has files check (fixes #15)

[markvandenbrink]

The has files check currently only checks the existence of a files property in the package.json file. But according to the npm docs you can also use a .npmignore file to do so:

You can also provide a .npmignore file in the root of your package or in subdirectories, which will keep files from being included.

This MR adds a condition to check if a .npmignore file is present so it can pass the test without a files property.