Application configurations load from defaults

I modified SkinnyConfig, it can load configurations from env block and root block in application.conf.

For example see below:
```
# default settings
app.setting = 100

# development
development {
  app.setitng = 200
}

# test
test {
}
```

``` scala
// skinny.env = 'development'
SkinnyEnv.stringConfigValue("app.setting") # => 200

// skinny.env = 'test'
SkinnyEnv.stringConfigValue("app.setting") # => 100
```