add support for function as plugin (#704)

Author: ambar

packages/razzle/config/runPlugin.js

@@ -6,6 +6,10 @@ function runPlugin(plugin, config, { target, dev }, webpack) {
     return runPlugin({ name: plugin }, config, { target, dev }, webpack);
   }
 
+  if (typeof plugin === 'function') {
+    return plugin(config, { target, dev }, webpack);
+  }
+
   if (typeof plugin.func === 'function') {
     // Used for writing plugin tests
     return plugin.func(config, { target, dev }, webpack, plugin.options);