Ignore invalid package.json files

[jan-molak]

Hi there and thanks for your work on credits-cli - it's a very useful module!

What I noticed when running credits-cli on one of my projects is that it struggles when it encounters an invalid package.json file. Sadly, some npm modules ship with test fixtures that include invalid package.json files (see npm/read-package-tree#18).

What I think would be useful is for credits-cli to simply ignore any invalid package.json files.

Thanks! Jan