Meta: adding new keys to the schema

I've changed the title to be a meta discussion, so that we don't have the same discussion in multiple issues:

- [optional flags](https://github.com/exercism/problem-specifications/issues/1492) #1492
- [immutability flags](https://github.com/exercism/problem-specifications/issues/1493#issuecomment-478563670) #1493
- [other types of tests](https://github.com/exercism/problem-specifications/issues/1225#issue-314244907) #1225
- multiline "type"
- [identity field](https://github.com/exercism/problem-specifications/issues/1473) #1473 (to avoid compromising [descriptions]( https://github.com/exercism/problem-specifications/pull/1525) #1525 to fit this purpose)

It seems there really is a lot of ask for new fields and there also seems to be a notion that we don't want to break the current generators.

Personally, I am currently parsing the comments for special behaviour and this means that my generators will all break when someone decides the comments should be a special format. I can derive no special meaning from the english in these comments. I don't think this is the right place to hold special cases, but it is for things not covered in the current scheme.

I know that you (@pgaspar @ErikSchierboom @coriolinus @sshine @SaschaMann and @NobbZ) are all involved with writing and maintaining the generators, or at least the discussions on GitHub:

- I think it would be wise to discuss first if we'll add new keys, if at all, because so far each issue has already raised the "yeah okay but we don't _really_ need it and it costs me work"-type comment
- ...and then which ones are candidates, so we can merge it at once and the burden on maintainers is much lower, as the raised point just mentioned _is valid_.
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Original issue about multiline.

There is canonical data such as [`food-chain`](https://github.com/exercism/problem-specifications/blob/master/exercises/food-chain/canonical-data.json#L4) and [`twelve-days`](https://github.com/exercism/problem-specifications/pull/954/files#diff-dcda8ef37a4982f40c53174662e64ebdR5) that correctly comment:

> JSON doesn't allow for multi-line strings, so all verses are presented
> here as arrays of strings. It's up to the test generator to join the
> lines together with line breaks.

Okay, that's great! But in this case I think it would be _great_ if we'd add a meta property (and keep a list of those, as there are more like these (such as a property preferably being a constant or at least immutable) so that we can automate generation of these exercises.