allow overriding Command builder short description with CMD_DESC variable -- close #22

spec/rscons/builders/command_spec.rb

@@ -1,16 +1,18 @@
 module Rscons
   module Builders
     describe Command do
+
       let(:command) { ['pandoc', '-fmarkdown', '-thtml', '-o${_TARGET}', '${_SOURCES}'] }
       let(:env) {Environment.new}
       subject {Command.new}
 
-
       it "invokes the command to build the target" do
         expected_cmd = ['pandoc', '-fmarkdown', '-thtml', '-ofoo.html', 'foo.md']
-        expect(subject).to receive(:standard_build).with("CMD foo.html", "foo.html", expected_cmd, ["foo.md"], env, :cache)
-        subject.run("foo.html", ["foo.md"], :cache, env, {'CMD' => command})
+        expect(subject).to receive(:standard_build).with("PANDOC foo.html", "foo.html", expected_cmd, ["foo.md"], env, :cache)
+        subject.run("foo.html", ["foo.md"], :cache, env,
+                    "CMD" => command, "CMD_DESC" => "PANDOC")
       end
+
     end
   end
 end