Bump version in multiple files, such as bower.json

Allows the `file` option to be specified as an array like `['package.json', 'bower.json']` to manage the versions in all of those files. This is useful for a library which is registered in both npm and bower.

This change is
- **Backwards compatible.** The `file` parameter can still be omitted or specified as a string.
- **Simple.** It introduces fewer changes than #25 while handling the most common use case.