Load the application's configuration from JSON files

Martini middleware that allows you to load the application's configuration from JSON files.

```go
func main() {
     m := martini.Classic()
 
     m.Use(config.File("config/main.json"))
 
     m.Get("/", func(conf config.Body) string{
         return fmt.Sprintf("Greeting: %s", conf["greeting"])
     })
 
     m.Run()
 }
```

I'am not sure  how much is this usable as middleware, because it runs on every request, but it's my try to learn how Martini works internally.

All suggestions are appreciated. 


Chreers