Map backend config for Logger

I want to write a file backend for the Logger.  
The backend config of Logger is a litte verbose. I must define a ID for backend and then use it to find out the specific config.

with this patch, I can do this
```
config Logger,
  backends: [
    %{mod: FileBackend, id: :error_log,  file: "error.log", size: 1024},
    %{mod: FileBackend, id: :debug_log, file: "debug.log", size: 10240},
  ]
```