In Futatabi, make it possible to set custom source labels.

An operator was consistently confused by having 3 on a left camera
and 4 on a right camera, and wanted L3 and R4 labels, so here goes :-)
There's no auto-import from Nageru at this point; it needs to be set
using --source-label NUM:LABEL (or -l NUM:LABEL) from the command line.