Improved passing of multiple postprocessor-args

Improved passing of multiple postprocessor-args

* Added `PP+exe:args` syntax
    If `PP+exe:args` is specifically given, only it used.
    Otherwise, `PP:args` and `exe:args` are combined.
    If none of the `PP`, `exe` or `PP+exe` args are given, `default` is used
    `Default` is purposely left undocumented since it exists only for backward compatibility

* Also added proper handling of args in `EmbedThumbnail`

Related: https://github.com/ytdl-org/youtube-dl/pull/27723