Deprecate the JSON command

[d0ugal]

Now that #457 has landed we should have people use the JSON it creates and remove the JSON command which is otherwise a bit weird.

Add a warning for 0.13
Louder warning in 0.14
Remove in 1.0

[d0ugal]

This will involve some work on the ReadTheDocs side - I wont remove this fully until we can get them transitioned over somehow.

[d0ugal]

This isn't actually fixed by 075410d. It'll be fixed when the command is fully removed.

[d0ugal]

Need to revisit this for 0.14.0