Support directly decoding nats body into heka message

Hrm, this isn't quite right. First, we'll want to take care of this in the Init() method, which used to be impossible but now is possible b/c a) config loads all of the decoders before any of the inputs and b) the WantsPipelineConfig interface makes pConfig (and thus PluginHelper) available before Init() is called.

Second, we don't want to rigidly hard code this. We want to inspect the decoder and then set a UseMsgBytes default value, but this value *should* be able to be overridden by a value explicitly set in the config. Which means that NatsInputConfig.UseMsgBytes will need to be a *bool instead of a bool so we can tell whether it was set or not.