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.

Since I needed to put a few commits for my idea use, the only commit I'm reference in this request is:
https://github.com/whentomanage/grunt-release/commit/0255ccf376bbfced9a861df148f82bb4cc0d4821

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: