Use JSON for labels/classes files

[widdowquinn]

The ad hoc tab-separated text format for label/class files works, but JSON may be more exchangeable and enhancement-friendly.

[baileythegreen]

@widdowquinn Can you elaborate, or say whether you still think this should be a goal?

[widdowquinn]

It is still a goal.

I would like us to move away from the restrictive, ad hoc current format of these files, to a more widely-used, readily-parsed format. There are several choices for a suitable format including JSON, YAML, TOML, and so on. We could prioritise something with a standard library parser in Python to minimise extra dependencies. It would be wise to choose something that we could extend, maybe to accommodate an array of labels, or other fields.