fix(controller): disallow custom optiomization (#578)

this will often break the inner workings of karma-webpack
due to a number of issues coming in about this in particular
it has been decided that it is best to ignore these kinds
of customizations and log a warning that they are discarded

Fixes #491

lib/karma-webpack/controller.js

@@ -33,6 +33,13 @@ defaulting ${newOptions.output.filename} to [name].js.`.trim()
       delete newOptions.output.filename;
     }
 
+    if (newOptions.optimization) {
+      console.warn(
+        'karma-webpack does not support custom optimization configurations for webpack, the optimizations passed will be ignored.'
+      );
+      delete newOptions.optimization;
+    }
+
     this.webpackOptions = merge(this.webpackOptions, newOptions);
   }