Multi-document streams

[tlsa]

Decide how/whether to support multi-document streams.

Maybe the current API only supports single document streams, and a new one can be added that takes a client callback that is called for each new document created from the stream.

[tlsa]

#12 fixes the issue where subsequent documents would trample the data for the first document. So with this, only the first document in the stream is loaded, and if there are more a warning is logged.