Add an option to IonSystemBuilder to allow the incremental reader to be used by the DOM

[tgregg]

Currently, any DOM method provided by IonSystem that internally creates an IonReader (e.g. singleValue, newLoader) uses a non-incremental reader. This could be changed by adding options to allow configuration of the IonSystem's internal IonReaderBuilder.

This would enable cleaning up some of the reader tests, e.g. BinaryTest.ion() and IonTestCase.load() (after merging #355).