Updates to the gif plugin

[seefood]

Description

• fixes for the latest versions of gifski and gifsicle commandline options.
• More warning messages from ffmpeg.
• print help when run with --help
• improved BASH syntax

Motivation and Context

updates since stale code was broken

How Has This Been Tested?

ran it locally with different options

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] If my change requires a change to the documentation, I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• [x] I have added tests to cover my changes, and all the new and existing tests pass.