Add configurable "stream_size" option.

This should give the user options to tune how responsive the stream reader is. For reading from file, a normal read is most optimal, but depending on what sentences are expected on a text string( i.e.: pyserial, tcp/ip socket) stream_size can be set appropriately. 

As expected for a stream_size smaller than a sentence, multiple next may need to be called to read in a full nmea_sentence.