adding support for addDefault

We are using `confit` to provide module config for kraken@1.0.0 middlewares. Idea is modules/middlewares can have default configuration which are environment based and app can override.

So the order of configs will be 
```
module/config.json
module/<env>.json
app/config.json
app.<env>.json
```
addDefault API is to initialize confit with app config folder and add module files as defaults.