fmt: operate on every kind of JSON file on a track

[ee7]

configlet fmt currently only operates on Concept Exercise and Practice Exercise .meta/config.json files.

It should also operate on the other JSON files:

• Approach config.json files
• Article config.json files
• The track config.json file
• Concept .meta/config.json files
• Concept links.json files

[ee7]

When fmt learns to format concept .meta/config.json files, I think we'll need to decide between:

• Adding a --concept option
• Renaming the fmt command's --exercise option to --slug, which can then be used for formatting an exercise or a concept (although this is probably bad if a concept can have the same name as an exercise).

We can also consider wheter configlet fmt should eventually grow some scoping options like sync, e.g.:

• --track
• --exercises
• --concepts

With the default of operating on all of them, as sync does.

[ErikSchierboom]

I'll take a look at this