Feature Request: support for partial JSON matching

Hello. First of all, great package!
It really reduces boilerplate code in tests and makes everything more readable.

I have a feature request/consideration I hope would get included to this project and I may help implement it if that's something that makes sense for this library: **Partial JSON matching**.

It can be behind a flag or in another method, but it basically means a JSON with 4 properties, when checked against a JSON with 2 properties, would not return the following error:

> expected 2 keys at '$' but got 4 keys
> unexpected object key(s) ["status","cause"] found at '$'

More often than not, we need to check for the overall structure and main fields and not the whole thing, and having to check for presence of every single field can take a lot of time.

Is this something that makes sense for you as the author of this library?
As much as I like creating my own fork, I think it's best for the community if we keep things centralized.

Thanks.

Start by reviewing linked pull request #42, since the issue names no files, tests, or entry points. Clarify the matching API and flag behavior there, then add coverage showing that extra JSON properties are accepted while the requested structure and fields still match.

Written by the indexing model from the issue text.