Add grunt-init-packagefile

[evanplaice]

Works the same way as grunt-init-gruntfile except it produces a package.json file.

grunt-init-node already does a great job of this but it works from the standpoint of scaffolding a new project.

grunt-init-packagefile would work from the standpoint of adding a package.json file to an existing project that's adding NPM support or projects that can't fall into the opinionated folder/file structure of a typical node application.