added functionality to ignore undefined options

[daanvdn]

hi @kohsuke, I added a small bit of functionality to CmdLineParser that allows to ignore undefined options, rather than throwing a CmdLineException. This behaviour can be controlled through the boolean switch org.kohsuke.args4j.ParserProperties#ignoreUndefinedOptions. This is switched off by default. Currently it only works for fields annotated with @Option and not with @Argument fields. I added two unit tests. If you would take this pull request into consideration, I'd be very grateful. Please let me know if any changes are required.