Allow users to set class name-ID mapping during export

[jveitchmichaelis]

This affects the COCO and Pascal VOC exporters. Currently the output class ID is dependent on the order in which they were created in deeplabel. It may be preferable to, in some cases, force this to something else, for example provide a pbtxt file like https://github.com/tensorflow/models/blob/master/research/object_detection/data/mscoco_label_map.pbtxt

The darknet exporter does not have this problem as it uses a .names file to control this. GCP is also not affected as it uses class names directly.

Should probably allow minimally:

• .names files, class name separated by newline
• .pbtxt files

and infer based on extension.