Support passing generic CUPS options to print jobs

[huerlisi]

First of all: great gem, loving it!

I've tried passing {'copies' => 40} to a .print_datacall, but got a message that this option is not supported by the printer.

Digging in the source code, I found out, that you validate the options against the PPD. But copies seems to be a generic option, available in all printers. Simply disabling the option validation did work: I've been able to pass the copies option and the printer respected it.

I'm no CUPS specialist and thus not sure if there are any other such generic options.

[huerlisi]

Thanks for merging my fix:-)