comma::command_line_options: add a check if an option is valid

[vlaskine]

i.e. something like:

comma::command_line_options options( ac, av, "--help,-h,--verbose,-v,--fields,-f" );

throw, if list is given and an option is not in that list

(the difficult part is to figure out whether it is an option or unnamed value: e.g there is no way to tell whether -1 is an option or an unnamed value)