Clean up NicoMovieMaker videos for external players

[AlexAplin]

NicoMovieMaker videos (nm#) may not be viewable in an external player. Videos with still images (e.g. nm11960162) will play fine with a pass through ffmpeg. Slideshows and GIFs (e.g. nm2829323) may play with improper frame timings or loops. Flash animations (e.g. nm4809023) will likely not play at all and may require other libraries or solutions.

[AlexAplin]

This has become less of an issue now that Nico has gone ahead and converted all of these videos to be viewable on the HTML5 player. Regardless, I ultimately think this issue is out of scope for this downloader as we are mainly interested in grabbing unaltered sources. I may revisit this in another project.