* support for non-contiguous SPNs (thanks j4l)
* can describe SPNs in transport layer in real-time (as their bytes are received) (thanks j4l)
* correctly reassembles RTS-CTS transport sessions as well (thanks j4l)
* can specify J1939 JSON db on command line (thanks j4l)
* default to describing transport PGNs as first-class PGN
* default to omitting description of incomplete frames