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, 15 insertions, 0 deletions

diff --git a/test/types_test.rb b/test/types_test.rb
index 44846b1..3696126 100644
--- a/test/types_test.rb
+++ b/test/types_test.rb
@@ -61,3 +61,18 @@ describe Slop::ArrayOption do
   end
 end
 
+describe Slop::NullOption do
+  before do
+    @options = Slop::Options.new
+    @version = @options.null('--version')
+    @result  = @options.parse %w(--version)
+  end
+
+  it 'has a return value of true' do
+    assert_equal true, @result[:version]
+  end
+
+  it 'is not included in to_hash' do
+    assert_equal({}, @result.to_hash)
+  end
+end