Introduce an option with string argument.
The string argument is put into `str_var`.
+`OPT_STRING_LIST(short, long, &struct string_list, arg_str, description)`::
+ Introduce an option with string argument.
+ The string argument is stored as an element in `string_list`.
+ Use of `--no-option` will clear the list of preceding values.
+
`OPT_INTEGER(short, long, &int_var, description)`::
Introduce an option with integer argument.
The integer is put into `int_var`.