Iterator that supports multiple items within files

Using a separate file for each individual exported item is fine until you get to really large numbers and want to batch multiple items into a file in order to reduce the overall number of files (for I/O performance purposes, for example).

I'm working on writing an iterator that supports a JSON encoded array within each file that gets iterated over by the Files iterator, including support for updating the progress bar correctly, and only passing one item to `process_item()` as expected.

Combined with #55 this allows for much more efficient use of files on the filesystem.