support for chunks

[4gra]

Chunked messages shouldn't require the user or the implementing command to have to negotiate multi-part messages, it should simply be detected and split as appropriate.

Messages which support / require chunk support should indicate this with $C or similar in the command json, so that the chunks can be auto-incremented (if a non-zero-terminated response is received?) and responses read - with the second chunk's prefix removed where appropriate.

This requires a new mode of interaction, a simple looped write / read / write which requires listening for a specific reply to a specific command, and therefore allowing read to give data back to the calling function rather than just spewing it to stdout.