URL list mode

[jayvdb]

Some of the other "pip offline" tools provide a list of URLs to-be downloaded/that were downloaded, or an install.sh (e.g. https://github.com/liu-hanwen/pip-offline).

This allows the set to be downloaded again on a different machine, without pip-downloader and the processing required, and also allows filtering of the URL list to exclude items which manual inspection deems to be unnecessary for the specific use-case.