Ability to set up pre-app handlers

[CWharton]

Trying to add CORS using 'github.com/rs/cors' as middleware but I am unable to get access to handlers through the middleware.

[markbates]

See #610 for the PR for this. Once this PR is merged you'll be able to write the following in the development branch:

func App() *buffalo.App {
	if app == nil {
		app = buffalo.Automatic(buffalo.Options{
			Env:         ENV,
			SessionName: "_coke_session",
			PreWares:    []buffalo.PreWare{cors.Default().Handler},
		})
		app.Use(middleware.SessionSaver)
		if ENV == "development" {
			app.Use(middleware.ParameterLogger)
		}
		if ENV != "test" {
			app.Use(csrf.Middleware)
		}
		app.Use(middleware.PopTransaction(models.DB))

		app.GET("/", HomeHandler).Alias("/foo")
		app.POST("/", HomeHandler)

		app.ServeFiles("/assets", packr.NewBox("../public/assets"))
	}

	return app
}

Using http://www.test-cors.org/#?client_method=GET&client_credentials=false&server_url=http%3A%2F%2Flvh.me%3A3000&server_enable=true&server_status=200&server_credentials=false&server_tabs=remote I'm able to test that the CORS library works.

This feature will be released in 0.9.4, but can be used today by switching to the development branch.

[wiemann]

FYI: This does enable CORS, but setting a header for the response a la c.Response().Header().Add("key", ''val") in the action produces a "http: multiple response.WriteHeader calls" warning.

[danikarik]

@wiemann Same issue. Results with "http: multiple response.WriteHeader calls" when calling non GET methods.