feat: allow frameworks to add preprocessors

This changes the order in which things are instantiated. Plugins are instantiated before preprocessors and all the other stuff required by `start` method. Thus a plugin can change `preprocessors` configuration before preprocessor consumes it.

lib/server.js

@@ -27,10 +27,6 @@ var log = logger.create();
 var start = function(injector, config, launcher, globalEmitter, preprocess, fileList, webServer,
     capturedBrowsers, socketServer, executor, done) {
 
-  config.frameworks.forEach(function(framework) {
-    injector.get('framework:' + framework);
-  });
-
   var filesPromise = fileList.refresh();
 
   if (config.autoWatch) {
@@ -311,5 +307,9 @@ exports.start = function(cliOptions, done) {
 
   var injector = new di.Injector(modules);
 
+  config.frameworks.forEach(function(framework) {
+    injector.get('framework:' + framework);
+  });
+
   injector.invoke(start);
 };