Include option to add file to existing ZIP

[Ecksters]

Would love an option to add new files to an existing ZIP file.

[nfriedly]

Yeah, that's not a bad idea. Is the behavior consistent across various zip implementations? If so, would you be willing to code it up along with some tests?

[Ecksters]

One issue I see is that Archiver doesn't appear to support it, so we'd potentially need to change the library used for fallback.

The native zip options pretty much universally support it.