Add a bump-only command

This adds the ability to do a two-step release, a la:

```coffeescript
grunt.registerTask 'publish', ['clean', 'bump-only', 'build', 'release']
```

This is an enviable feature of `grunt-bump`. Would also resolve #53

There weren't many tests to code against, so I'd love some help on that front in case I missed anything obvious.