Refactored @-options handling to save bytes.

If an option is set to @, the global option of the same name as the key
will be used, e.g.:
acceptFileTypes: '@', // acceptFileTypes

If the options set contains a property prefix, the global options key
will be the prefix plus the option key, concatenated in camel-case
spelling, e.g.:
prefix: 'loadImage',
acceptFileTypes: '@', // loadImageAcceptFileTypes

If the prefix is set to true, the action of the options set will be
used as key.