Add Sass like extend

lib/less/tree/ruleset.js

@@ -23,6 +23,12 @@ tree.Ruleset.prototype = {
         // push the current ruleset to the frames stack
         env.frames.unshift(ruleset);
 
+        // currrent selectors
+        if (!env.selectors) {
+            env.selectors = [];
+        }
+        env.selectors.unshift(this.selectors);
+
         // Evaluate imports
         if (ruleset.root || ruleset.allowImports || !ruleset.strictImports) {
             ruleset.evalImports(env);
@@ -59,6 +65,7 @@ tree.Ruleset.prototype = {
 
         // Pop the stack
         env.frames.shift();
+        env.selectors.shift();
         
         if (env.mediaBlocks) {
             for(var i = mediaBlockCount; i < env.mediaBlocks.length; i++) {