option-refSearch options for a command line option named key and
return its value, if found. If the option has no value, but was given,
return #t. If the option was not given, return default.
options must be the result of a call to getopt-long.
option-ref always succeeds, either by returning the requested
option value from the command line, or the default value.
The special key '() can be used to get a list of all
non-option arguments.