Assign default value to `:listing` option

When you create a class command, there is a problem with `:listing`
option, which doesn't carry the correct default value. Consider the
example:

  class MakeMeHappy < Pry::ClassCommand
    match 'woot'
  end

`MakeMeHappy` command matches against 'woot' String, but its `:listing`
option is set to the "nil" String, which is incorrect. We can fix it by
setting `:listing` explicitly:

  command_options :listing => 'woot'

It's a repetitive task, so we can automate it. Holy smoke, this why we
all use computers, after all!

With help of this commit there is no need to set `:listing` manually.
Pry will handle it for you.

Signed-off-by: Kyrylo Silin <kyrylosilin@gmail.com>