Convert the option’s value type.
The conversion is called once for each value in each invocation of the option. If any errors are thrown,
they are caught and a BadParameterValue is thrown with the error message. You can call
fail to throw a
metavar - The metavar for the type. Overridden by a metavar passed to option.
envvarSplit - If the value is read from an envvar, the pattern to split the value on. The default
splits on whitespace. This value is can be overridden by passing a value to the option function.
completionCandidates - candidates to use when completing this option in shell autocomplete,
if no candidates are specified in option