Added event OptionParsed on OptionSet

Added a new event "OptionParsed" on OptionSet that fires whenever one of the options it contains has finished parsing, containing the parsed OptionContext.
The whole reason `Option` has the `Parsed` event is so that `OptionSet` can hookup to it. This isn't necessary.

With an `internal` `OptionSet.OnOptionParsed()` method, you can just invoke it from here:

    c.OptionSet.OnOptionParsed (new OptionParsedEventArgs (c));