fix(preprocessor): do not show duplicate warnings

lib/preprocessor.js

@@ -11,9 +11,10 @@ var sha1 = function(data) {
   return hash.digest('hex');
 };
 
-
+// TODO(vojta): instantiate preprocessors at the start to show warnings immediately
 var createPreprocessor = function(config, basePath, injector) {
   var patterns = Object.keys(config);
+  var alreadyDisplayedWarnings = Object.create(null);
 
   return function(file, done) {
     var preprocessors = [];
@@ -27,16 +28,21 @@ var createPreprocessor = function(config, basePath, injector) {
       preprocessors.shift()(content, file, nextPreprocessor);
     };
     var instantiatePreprocessor = function(name) {
+      if (alreadyDisplayedWarnings[name]) {
+        return;
+      }
+
       try {
         preprocessors.push(injector.get('preprocessor:' + name));
       } catch (e) {
-        // TODO(vojta): log warning only once per each preprocessor
         if (e.message.indexOf('No provider for "preprocessor:' + name + '"') !== -1) {
           log.warn('Can not load "%s", it is not registered!\n  ' +
                    'Perhaps you are missing some plugin?', name);
         } else {
           log.warn('Can not load "%s"!\n  ' + e.stack, name);
         }
+
+        alreadyDisplayedWarnings[name] = true;
       }
     };