feat: add arg `--img-lib`. This allows the user to select one of the two image libs, defaulting to fpng.
refactor: improved error handling
feat: added temp directory clean up even if the program crashes
refactor: removed --index-dir arg
feat: added --source-index-path and --encode-index-path, these are direct paths to the index file vs only having a directory to search
refactor: added a more descriptive help message for --subtitle-color
