Load config to change class name for current

## Ex) Class name for Twitter bootstrap

```
{
    "current_page": {
        "classname": "active"
    },
    "plugins": {
        "helpers": {
            "current_page_helper": "punch-current-page-helper"
        },
        "template_engine": "punch-engine-handlebars"
    }
}
```

I have no idea how to test loading config, so there is no test sorry.
If it really required, I will try.
I'm sure no error rised when key of `current_page` is not exists in config.json
because I ran this code on my project.