Add server plugin support

Allows a simple plugins array to be specified in testem config. Each plugin is `require`d in as a node module, and an init method is called, passing the plugin configuration and the Express server.

This allows the user to create a simple Node module that can call `app.get`, `app.post`, `app.use` to configure customized routes and middleware.

One part that I wasn't quite sure about is the error logging. It might be helpful to have slightly more robust error-handling and messages when a plugin configuration fails, but I wasn't sure what would be the best way to do that. I'd love to hear some suggestions.