npm version updates shrinkwrap if config set.

Currently when you run `npm version`, it will update the version in package.json, commit the change, then tag it. This is awkward if you're using shrinkwrap, because the shrinkwrap file also sets a version.

This allows you to set a npm config attribute `version-shrinkwrap`. If this flag is set, when you run `npm version`, the shrinkwrap will also be updated.