Default Value support

Hi,

Thanks a lot for this gem!

I wrote a little hacky class that adds support for default values. So you can do:

parser.on('-m', '--tracing-mode MODE', 'Define the category of events traced', default: 'default')

I have no idea if you can find it interesting, but I use it quite often in my project, so I share it with you. Putting the default values into the into for me to duplicate the key (one into the the on and one into the into. I lead from some silly mistake due to a typo).

If you are interested, maybe I can try to do a cleaner PR. If you are not, please feel free to close this issue.

Thanks a lot!

require 'optparse'
class OptionParserWithDefaultAndValidation < OptionParser
  def initialize
    # Dictionary to save the default values passed by `on`
    @defaults = {}
    super
  end

  def parse!(argv = default_argv, into: nil)
    # Merge the default value with the `into` dict
    into.merge!(@defaults) unless into.nil?
    # This will populate the `into` dict (overwriting our default if needed)
    super
  end

  def define(*opts, &block)
    switch = super
    @defaults[switch.switch_name] = @tmp_default unless @tmp_default.nil?
    switch
  end

  def on(*opts, default, &block)
    # Save the default, which will be used in `refine.`
    @tmp_default = default
    # Default can contain Array (or Set).
    # We cannot use the [a].flatten trick.
    # So we use the respond_to one
    #   irb(main):028:0> default = Set[-1,2]
    #   => #<Set: {-1, 2}>
    #   irb(main):029:0> [default].flatten.join(',')
    #   => "#<Set: {-1, 2}>"
    #   irb(main):030:0> default.respond_to?(:flatten)? default.flatten.join(',') : default
    #   => "-1,2"
    opts << "Default: #{default.respond_to?(:flatten) ? default.flatten.join(',') : default}" unless default.nil?
    super(*opts, &block)
  end
end

Start with the OptionParser#on, #define, and #parse! entry points shown in the proposed subclass, and review how the existing into argument is populated. Confirm the expected behavior for default values with and without into, then locate the existing OptionParser tests and use them to define completion criteria.