Added JSON schema for v.1.1

[anotherpit]

Having full spec as a text is cool, but having it as a JSON schema also gives you validation, autocomplete and auto documentation in modern IDEs.

Things to note:

• Suggested JSON schema is permissive rather than restrictive: it only cares about known properties, and ignores unknown ones. Otherwise, some of the current examples in the repo would become invalid (e.g. diagonal-folded.fold contains unspecified vertices_edges property).
• If you accept this PR, it's a good idea to publish the scheme, so that we can have a public URL (which should replace relative value of $scheme in the box-with-scheme.fold example), that people can use without downloading the schema from the repo.