feature: support longer lists of input files

[roycewilliams]

When using Gifski for (silly) purposes that require large lists of image files, they cannot be passed to Gifski by Linux shells, producing the usual "Argument list too long" error.

Alternatives might be things like "read all image files in this directory" or "read image filenames from this file".

[MrDrMcCoy]

As a workaround for this, you can use the "no temp files" method for this:

ffmpeg -framerate 30 -pattern_type glob -i '*.png' -f yuv4mpegpipe | gifski -o "crushed.gif" -

[roycewilliams]

@MrDrMcCoy Interesting workaround! Is the glob expansion order predictable and consistent?

[MrDrMcCoy]

I'm not sure on that, and have seen conflicting anecdotes in various places. However, there's another method that's completely unambiguous: https://superuser.com/a/1501040