Apply additional arguments to :cmd after defaults.

lib/guard/rspec/command.rb

@@ -21,6 +21,7 @@ def _parts
         parts << _visual_formatter
         parts << _guard_formatter
         parts << "--failure-exit-code #{FAILURE_EXIT_CODE}"
+        parts << options[:cmd_additional_args] || ""
         parts << paths.join(' ')
       end
 

lib/guard/rspec/options.rb

@@ -8,6 +8,7 @@ module Options
           failed_mode:     :none,  # :keep and :focus are other posibilities
           spec_paths:      %w[spec],
           cmd:             nil,
+          cmd_additional_args: nil,
           launchy:         nil,
           notification:    true
       }

spec/lib/guard/rspec/command_spec.rb

@@ -50,6 +50,14 @@
         expect(command).to match %r{-f doc}
       end
     end
+
+    context "with cmd_additional_args" do
+      let(:options) { { cmd: 'rspec', cmd_additional_args: '-f progress' } }
+
+      it "uses them" do
+        expect(command).to match %r{-f progress}
+      end
+    end
   end
 
 end