Simpler property mapping

It would be nice if properties could be mapped a bit more dev friendly.

For the current version (0.0.11) you need to define everything in `propertyTransform`, like 

```javascript
const mapper = new Mapper( {propertyTransform: p => p === 'foo' ? 'foozed' : p} )
```

As Devs are lazy, I should provide a neater way to do that. 🍨 