Potential new option for "Download remote assets"

[Alarich]

Would it not be feasible to have an option in the converter setup screen to "Download remote assets".

This related to #45, where you could use the --json option to download every file.

Then you would get the best of both worlds - you can convert the existing files from the zip file and if the converter finds an URL - it will download the file instead of ignoring it or replacing it with mystery-man.svg for example.