Add NullOption and default to using it

A NullOption is one whos return value we don't care about. For example,
you might just want a `--version` option which simply prints the
version and exits. In this case, having a `true` value in `to_hash` is
really just noise. We probably don't care about it, using NullOption
discards it.

I think using this for Options#on makes sense because this is logical:

    opts.on '--version' do
      puts VERSION
      exit
    end

Rather than:

    opts.add '--version' do
      puts VERSION
      exit
    end

And defaulting to a StringOption. This also means you have to be
explicit about adding such an option, which is a good thing.

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/slop/option.rb b/lib/slop/option.rb
index 089d36b..85d7cb7 100644
--- a/lib/slop/option.rb
+++ b/lib/slop/option.rb
@@ -53,6 +53,12 @@ module Slop
       true
     end
 
+    # Override this if you want to ignore the return value for an option
+    # (i.e so Result#to_hash does not include it).
+    def null?
+      false
+    end
+
     def value
       @value || default_value
     end