Add support for ZIP file attachments.

[mbostock]

Adds fileAttachment.zip which returns a promise to a ZipArchive. The ZipArchive exposes an archive.fileNames() method which returns the array of file names, and an archive.file(name) method which returns a FileAttachment-like object. So, to load a CSV file in a ZIP file, you can now say:

archive = FileAttachment("ACS_14_5YR_B01003.zip").zip()

data = archive.file("ACS_14_5YR_B01003_with_ann.csv").csv({typed: true})

As with XLSX #215, I’ve decided not to expose JSZip as a recommended library, and instead wrap it with a more minimal abstraction that feels more Observable-ish.

[mbostock]

I assume for archives with directories, the filenames array will contain values like "dir/file.csv"? and only contain files and not "dir/"?

Good catch. It did include directories, but now I’ve filtered them out.