Allow specifying multiple inputs in inputpath

[ItsABlackScreen]

Problem
Currently inputpath and outputpath must both be either dirs or images.
This leads to the situation that to upscale 2 images from a directory that may contains 100's either the 2 images must be moved to a separate directory or the program called multiple times with each image as the inputpath, which is a lot slower than batch processing of multiple images.

Proposal
Allow inputpath to be specified with any number of paths that then output to a given directory.

Example
waifu2x-ncnn-vulkan -i "01.jpg" "02.jpg" -o /path/to/output/dir -f png

Which then produces:
dir/01.png
dir/02.png