Translate root x- extensions into extension element

[kylef]

We need to create a specification for this, we proposed in past to move it to apielements.org website.

[kylef]

There is some design problems with the way that Swagger 2 adapter implements this which we should address in specification (https://help.apiary.io/profiles/api-elements/vendor-extensions/).

The open questions:

• How should object be serialised? Swagger 2 adapter and extension spec incorrectly makes this into a object not object element.
• How should we map extensions to elements? Some categories of extensions either overlap or do not map to API Elements. For example info and "root" extension.