Don't clobber pre-existing options when adding new ones

lib/pry/command.rb

@@ -27,7 +27,8 @@ class Pry
 
       # Define or get the command's options
       def command_options(arg=nil)
-        @command_options = arg if arg
+        @command_options ||= {}
+        @command_options.merge!(arg) if arg
         @command_options
       end
       # backward compatibility

test/test_command.rb

@@ -223,6 +223,16 @@ describe "Pry::Command" do
 
       mock_command(cmd, %w(--four 4 four))
     end
+
+    it 'should allow overriding options after definition' do
+      cmd = @set.command_class /number-(one|two)/, "Lieutenants of the Golgafrinchan Captain", :shellwords => false do
+
+        command_options :listing => 'number-one'
+      end
+
+      cmd.command_options[:shellwords].should == false
+      cmd.command_options[:listing].should == 'number-one'
+    end
   end
 
   describe 'tokenize' do