Consider adding support for `Diff`

[thomas11]

The Diff operation is defined in the Pulumi resource model.

The Diff method is responsible for calculating the differences between the actual and desired state of a resource as represented by its last recorded state and new input Object as returned from Check or Read and the logical operation necessary to reconcile the two (i.e. no operation, an Update, or a Replace).

Allowing users to define a specific command to run on Diff could open up new use cases. If you have such a case, please comment below!

This issue is a follow-up from #20.