Updated plugin import syntax

- Updated parser to recognize `@plugin` and removed parsing support for
the (plugin) import option.
- Updated plugin import unit tests to `@plugin` syntax

lib/less/parser/parser.js

@@ -1212,7 +1212,7 @@ var Parser = function Parser(context, imports, fileInfo) {
             },
 
             importOption: function() {
-                var opt = parserInput.$re(/^(less|css|multiple|once|inline|reference|optional|plugin)/);
+                var opt = parserInput.$re(/^(less|css|multiple|once|inline|reference|optional)/);
                 if (opt) {
                     return opt[1];
                 }
@@ -1290,6 +1290,41 @@ var Parser = function Parser(context, imports, fileInfo) {
                 }
             },
 
+            //
+            // A @plugin directive, used to import compiler extensions dynamically.
+            //
+            //     @plugin "lib";
+            //
+            // Depending on our environment, importing is done differently:
+            // In the browser, it's an XHR request, in Node, it would be a
+            // file-system operation. The function used for importing is
+            // stored in `import`, which we pass to the Import constructor.
+            //
+            plugin: function () {
+                var path,
+                    index = parserInput.i,
+                    dir   = parserInput.$re(/^@plugin?\s+/);
+
+                if (dir) {
+                    var options = { plugin : true };
+
+                    if ((path = this.entities.quoted() || this.entities.url())) {
+
+                        if (!parserInput.$(';')) {
+                            parserInput.i = index;
+                            error("missing semi-colon on plugin");
+                        }
+
+                        return new(tree.Import)(path, null, options, index, fileInfo);
+                    }
+                    else
+                    {
+                        parserInput.i = index;
+                        error("malformed plugin statement");
+                    }
+                }
+            },
+
             //
             // A CSS Directive
             //
@@ -1301,7 +1336,7 @@ var Parser = function Parser(context, imports, fileInfo) {
 
                 if (parserInput.currentChar() !== '@') { return; }
 
-                value = this['import']() || this.media();
+                value = this['import']() || this.plugin() || this.media();
                 if (value) {
                     return value;
                 }

test/less/import-plugin-scoped.less

@@ -1,5 +1,5 @@
 .in-scope {
-  @import (plugin) "./plugins/test";
+  @plugin "./plugins/test";
   result : test();
 }
 

test/less/import-plugin.less

@@ -1,4 +1,4 @@
-@import (plugin) "./plugins/test";
+@plugin "./plugins/test";
 
 .test {
   result : test();