Config file load for custom

Config files:
```bash
config/application.conf
config/custom.conf
```

Create config class:
```D
module app.config.CustomConfig;

class CustomConfig
{
    // define some config items
}
```

Get config object:
```D
// Get config/application.conf
auto appConfig = app().config();

// Get config/custom.conf
auto customConfig = app().config!CustomConfig("custom");
```