Get label map from tflite file

[iingram]

README suggests manually constructing the label map. This can be done more easily and automatically by extracting it from the metadata in the tflite file by unzipping the tflite file. README (and perhaps relevant code) could be updated to take advantage of that.

[iingram]

README addresses this now. Maybe just do the unzipping in the training script so user doesn't have to run those extra steps manually?