Added more ffmpeg support, qscale:v, vf fps, and duration in that order.

[Da-Scher]

Hello wallset! not sure if these changes are outside of the spirit of the script, but I added some features that can improve the quality of images in the _video() and _set_video() functions. Example: $ wallset -V hello_world.mp4 2 25 8 where the 3rd argument is the quality scale, 4th argument is the frame rate, and 5th argument is the duration.