Report totals

[dblock]

I'm using this tool to report API coverage in the OpenSearch API spec project. The workflow reports something like "API specs implemented for 232/649 (35%) APIs". The way it works is that we have a way to dump the number of APIs, then compare the spec.

I propose openapi-diff reports some counters into the output to make this process easier, such as totals and % of differences, possibly at multiple levels in attributes.