Fixes an issue where feature states are being created without their initial states

[congwenma]

This addresses the problem that whenever you use forFeature you must explicitly add an initialState

StoreModule.forFeature('cars', carReducer, {
  metaReducers,
  // This was implicitly required
  initialState: carInitialState 
})

Should fix:

#96
#82

@ernestomancebo @btroncone @bufke Thoughts?