Feature: Add ability to append to index file

Right now, all your packages must be in your `--package-path` directory when you build your index. I'd like to add the ability to build your index with only new packages in `--package-path`. This way, you don't have to keep around the `--package-path` directory and can use releases as the sole source of truth.

I can write in this functionality, I just want to make sure this is a welcome addition first.