Allow incremental processing of documents

[kpodsiad]

According to the spec YAML was designed to support incremental interfaces that include both input (“getNextEvent()”) and output (“sendNextEvent()”) one-pass interfaces.

However, the current implementation allows only batched processing of whole documents. This change would be also very handy when working with the official test suite because it has negative tests, for instance this one. Those test cases contain a list of expected events before an error occurrence.