Adding ability to npm shrinkwrap as part of release

I assume you won't take this pull request as is, but I'd love to get the ability to [npm shrinkwrap](https://npmjs.org/doc/shrinkwrap.html) into this package.

With this addition, the grunt-release is perfect for my workflow. What can I tweak (besides adding tests), for you to accept this?

Thanks for the repo! :+1: