Need attribute support

[timjr]

It's not clear how to do attributes like Vatic supports. For example, if I annotate a traffic light, I might like to mark its color. If I use different labels, like trafficlight-red, trafficlight-green, then they will not be the same object ID, so I would have to merge the tracks. It is better to have a color attribute in addition to the label that can be red or green.

[antingshen]

There's not a feature to do this at the moment. Happy to help look over things if you'd like to add it though :)

[clayshieh]

122

[jeanineharb]

Pull request #128 answers this need.