Needs a test. And it's probably fine to just use one config value for both?

Otherwise LGTM, once the linter agrees.

During testing, I found out that stringify-package does not accept indent value 0 due to the lack of strict comparison. I've created a PR to stringify-package, if that PR will be accepted this PR will work.

Also I discovered that in function lib/shrinkwrap.js : checkPackageFile files package-lock.json and npm-shrinkwrap.json were processed using name package.json and package.json itself was processed (stringified) in function lib/install/save.js : savePackageJson so I removed condition (name === PKGLOCK || name === SHRINKWRAP).