Allow users to pass in case class objects containing variable mappings

[vlthr]

Levee currently accepts a Map[String, Any] containing variable bindings, where the values in the map are converted to Value objects using Value.create to provide the Map[String, Value] that Levee uses internally. This works, but relies on ugly casts and can be hard to use.

This could be improved by instead taking case class objects providing the mappings, and automatically generating the Map[String, Value] using an LabelledGeneric[T] in Shapeless.

Reliability could also be improved by using Typeable instead of raw type casts in Value.create.