Converter options definitions are now exposed

• Converter option definitions are now accessible along with suggested UI and helptexts. This allows one to auto-generate a frontend based on conversion options. The feature is already in use in the webp-convert file manager, which is used in WebP Express. New method: WebPConvert::getConverterOptionDefinitions()
• The part of the log that displays the options are made more readable. It also now warns about deprecated options.
• Bumped image-mime-type guesser library to 0.4. This version is able to dectect more mime types by sniffing the first couple of bytes.
• And more (view closed issues here