Correctly calculate $VIDEO_NAME macro for URL inputs

[Breakthrough]

Currently, the macro $VIDEO_NAME is not calculated correctly when using ~an image sequence (e.g. img_%04d.jpg) or~ [1] a URL as an input video, leading to (potentially) malformed output filenames when running other commands that create output files on disk (e.g. list-scenes). This only happens when the user does not override the calculated filename.

Need to detect this case and either error out while parsing (forcing the user to provide an explicit filename), or output a warning suggesting the filename be overridden manually.

[1] Completed in v0.6

[Breakthrough]

This should work correctly now for image sequences as of v0.6, but URLs still need to be tackled.