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);