Allow pa11y-ci to merge existing pa11y json reports

[simonedavico]

My use case: I am running pa11y through a custom Cypress command, so that I can have a unified API to interact with/navigate through pages and extract a11y reports.

For each a11y test case, I save pa11y output to a JSON file.

I would like to be able to call pa11y-ci by passing a list of json report paths generated by pa11y, to obtain a final report equivalent to what I would get by running pa11y-ci -j.

The API could work like this:

npx pa11y-ci -j --merge ./report/home.json ./report/about.json 

It should be easy to extract the required aggregation logic from the processResults function.

Is this something you would consider for pa11y-ci? If yes, I can have a go at implementing it.