Add URLs support to collection

[rgaudin]

To support nautilus-webui but also because it makes nautilus a lot more flexible, we want the JSON collection to support setting URLs on each entry in addition to files.

Because the simple List[str] for files is already showing limits (#57), we want to extend it.

• It's still a list
• It can still contain a single str and in this case, current behavior applies (it's a member name of the accompanying archive)
• It can also be a Dict[str, str] composed of

{
  "archive-member": "01 BOOK for printing .pdf",  # optional, member name inside archive (same as current)
  "url": "http://books.com/310398120.pdf",  # optional, has precedence over `archive-member`, url to download file from
  "filename": "My book.pdf",  # optional, filename to use in ZIM, regardless of original one
}

One of archive-member or url should be present, otherwise it is not considered.
All those url are to be fetched and stored in the ZIM, at the requested filename or using the basename of the URL.

User is responsible for not uniqueness of filenames.