Add analysis of specification extensions

[SensibleWood]

Some analysis of specification extensions would be very useful.

Collecting information on where API providers repeatedly implement a specification extension could help guide the future enhancements to the OpenAPI specification. Whilst this might prove spurious in a single OpenAPI document or for a single API provider it may prove noteworthy across many API providers. This is especially true when one considers that specification extensions can be used in most properties in OpenAPI. Having such an understanding could therefore be very informative.