Simpler property mapping

[ohager]

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

const mapper = new Mapper( {propertyTransform: p => p === 'foo' ? 'foozed' : p} )

As Devs are lazy, I should provide a neater way to do that. 🍨