Add "max" level alias; various level-related warnings

[RReverser]

• Show a deprecation warning for Options::from_preset(4) / -o 4 as per Is preset 4 deprecated or incorrect? #221. Can be still removed later, but for now at least avoids confusion.
• Show warning for usage of levels 4 and above for Zopfli and Libdeflate. This is not immediately obvious to users, but such levels end up iterating over same parameters as level 3 for these compressors, since there are no fine-tuned compressor options.
• Adds Options::max_compression and -o max to always refer to best compression as discussed in Is preset 4 deprecated or incorrect? #221.
• Warns about usage of levels that don't exist yet. Previously these would silently downgrade to -o 6, but I think it's worth showing a warning now that we have explicit max_compression method / option.