Adding the capability to pack WEBP images into compiled assets catalog.
Since this is not standard behavior, I'm adding it under `--allow-image-type webp`.
This way (`--allow-image-type`) also makes it easier to add future new extension types.
Since this is not a feature supported by Apple's Xcode.actool, I'm adding `--allow-non-standard-behavior` as well, that makes it explicit it's incompatible behavior.
Asset catalog compilation now uses a custom variable to pass inputs
to the specification, and a custom grouping method to group inputs.
Does not pass sticker pack translations to asset catalog compiler,
as this seems to require parsing the asset catalog just to determine
which strings files should be used. Left as a future task.
Instead of checking if strings are empty, use optionals to store if
the arguments were passed at all. Additionally, add a few utility
methods to the option parser to simplify various tool option classes.
The contents printing supports most types of asset, but additional ones
can be supported more in the future. Output can be human-readable text or
as a property list, as specified by the output format.
