Deprecate --input-directory in favor of --input-files

[CodeF0x]

Right now, users must use --input-directory to specify all input files. The name is misleading though, as you can not supply a directory which ffzap will automatically get every eligible file from. Instead, you need to supply every file name individually or use a glob pattern (or the respective equivalent on other systems).

I propose --input-files or maybe just --input.

That's an opportunity to also adjust --input-file as its name is also misleading. You're not inputting a file, but instead a text file containing a list of files to process.

I propose --files-list or something along the lines.

[CodeF0x]

Settled on --input as a replacement of --input-directory and --file-list as a replacement of --input-file.