start refactoring toCSS so we will be able to collect sourcemap information at the same time

lib/less/tree/value.js

@@ -17,6 +17,9 @@ tree.Value.prototype = {
             }));
         }
     },
+    genCSS: function (env, output) {
+        output.add(this.toCSS(env));
+    },
     toCSS: function (env) {
         return this.value.map(function (e) {
             return e.toCSS(env);