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

[jsmorph]

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.

[jsmorph]

To be clear: The issue is for this reference (Go) implementation. A specification for specs could be silent on this property -- or just suggest it.

[jsmorph]

What's a better name for the property? specFormat isn't quite right. We're not talking about JSON, YAML, or other rendering syntaxes ...

[westokyo]

It is my opinion that the version number must be required. The specification definition is independent of any implementation. Thus any current, or future, library needs to know how to read the specification. The version number provides that ability.

[jsccast]

sheensVersion instead of specFormat? An advisory, optional property, which takes a string value.

Aside: This information could be provided in an envelope. Since presumably some applications will have different ideas about how (and if) this information is used (and even represented), it's polite and economical to avoid forcing one approach or another. In a related spirit, if we want to encourage (or even allow) an application to inline application-specific properties in a spec, we should provide some cheap namespacing. Perhaps even just "a property with a name of this form is reserve for this or that". Maybe later.

Since the sheensVersion information probably does have a good bit of demand, we can just take the expedient approach of offering applications the use of that specific property if and when they want.

[jsccast]

Probably need to change this (reference) implementation to have that property in the Spec struct.