Add property to spec to represent the version of the spec structure

This data would represent the version of the spec _structure_, not the version of the spec itself. For example, in Sheens 1.0, the spec structure version might just be that same `1.0`. My joke-telling spec, which is written for (say) `specFormat: "1.0"`, could have `"version":"3.4.5"`.

So the proposal here is to add `specFormat`, with string values, as an optional spec property.

Also see #35.

No files, tests, or entry points are named in the issue. Read issue #35 first, then locate the repository's spec representation and validation paths; done means specs can optionally carry a string-valued specFormat that distinguishes structure versions from spec versions.

Written by the indexing model from the issue text.