Obtains the value of the specified built-in or custom option.
option_name: String value of the built-in or custom option.
Returns one of the following, depending upon the attributes defined for the option:
- Return value of func (after execution) if a func has been associated with the option; or
- Default value, if one has been specified for the option; or
- Value of the option as passed to the script; or
- Undef, if none of the above have been specified.