Added a command builder to the list of default builders

spec/rscons/builders/command_spec.rb

@@ -0,0 +1,16 @@
+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})
+      end
+    end
+  end
+end