Generalize for other bulk operations #19

The pyesbulk package is currently hardcoded for "create" operations; we would like to be able to leverage the retry logic for "update" and "delete" as well. This requires some refactoring, adding a kwarg to the external entry points to allow overriding the global _op_type or, better, building a Pyesbulk object that could be instantiated to override this and perhaps other global assumptions.