waifu2x-ncnn-vulkan multi-gpu support

[lovesegfault]

Description

While waifu2x-ncnn-vulkan supports multi GPU, video2x's current argument parser makes it so that this does not work, as noted in #115.

I'd like to be able to pass multi-gpu args to waifu2x-ncnn-vulkan, such as -g 0,1, -t 400,400, -j 1:1:1,1:1:1.

Sources

#115