WIP Implement ClumpSource.apply(input) to allow strict fail-if-not-found behaviour

@fwbrasil there may be a more elegant way to implement this aside from passing a Boolean around. 

I thought it might be nice to get people the option to EXPECT keys to be found and fail-fast if they are not. Following the semantics from scala `Map`, `apply` is strict, `get` returns an `Option`

Possibly I should be adding unit-tests for these too, currently I only modified the integration spec. 

WDYT?