fix: support rxjs lettable operators (#307)
fix: support rxjs lettable operators (#307)

Fixes #247
DavidParks8/ng-packagr,DavidParks8/ng-packagr,DavidParks8/ng-packagr,DavidParks8/ng-packagr,dherges/ng-packagr,dherges/ng-packagr,dherges/ng-packagr,dherges/ng-packagr