Allow matching against partial expectations

[Szasza]

Hi @gusaul ,

Thank you for maintaining this package, it is much appreciated.

It may be a good feature to add matching based on partial expectations. An example is when there is an item gets put into a DynamoDB table with a timestamp, which timestamp is inaccessible from the tests given that it is part of the function one wants to test.

If there was a possibility to do partial matching besides the currently existing deep equality check in the PutItem function, one could handle the above use-case.