Fix addOption() method

After trying out things on multiple printers I realised that the addOption() method does not work. It was not too straightforward to find a reference of what the num_options argument of the cupsAddOption mean. I found out that it is the current number of options currently in the options array. So I made these variables global per ofxCUPS class instance.

Additionally I added a parseOptions() method to make use of the command-line reference found here: http://www.cups.org/documentation.php/options.html