add preprocessors to plugins

less/less-tests.ts

@@ -11,3 +11,27 @@ less.render("fail").then((output) => {
 }, (error: Less.RenderError) => {
     console.log("rejected as expected on line number " + error.line);
 });
+
+var preProcessor: Less.PreProcessor = {
+    process: (src, extra) => {
+        console.log(extra.imports, extra.context);
+        if (extra.fileInfo.filename === "foo.less") {
+            return ".other-rule { width: (1 + 1); }\n " + src;
+        } else {
+            return src;
+        }
+    }
+};
+
+var myPlugin: Less.Plugin = {
+    install: (less, pluginManager) => {
+        alert(less.version[2]);
+        pluginManager.addPreProcessor(preProcessor, 1000);
+    }
+};
+
+var options: Less.Options = {
+    plugins: [myPlugin]
+};
+
+less.render("h1 { background: red; }", options);

less/less.d.ts

@@ -30,12 +30,30 @@ declare module Less {
 
     class PluginManager {
         constructor(less: LessStatic);
+
+        addPreProcessor(preProcessor: PreProcessor, priority?: number): void;
     }
 
     interface Plugin {
         install: (less: LessStatic, pluginManager: PluginManager) => void;
     }
 
+    interface PreProcessor {
+        process: (src: string, extra: PreProcessorExtraInfo) => string;
+    }
+
+    interface PreProcessorExtraInfo {
+        context: {
+            pluginManager: PluginManager;
+        };
+
+        fileInfo: RootFileInfo;
+
+        imports: {
+            [key: string]: any;
+        };
+    }
+
     interface SourceMapOption {
         sourceMapURL: string;
         sourceMapBasepath: string;