Ideas for middleware.

[pksjce]

Kudos to you for making this lib! I'm loving the mutating abilities.
I want to talk about having some middleware like abilities or hooks to the update function to implement common functionality among all mutations.
Two use cases that I wanted particularly were

1. Logger/debug tool
2. Crash reporter.

Did you have any plans of supporting such a thing? Would you be interested in a PR if I worked on it?

[aweary]

I'm am definitely in favor of supporting middleware! Thanks for opening this issue, and a PR. I'll review it as soon as I can.

[pksjce]

Cool! The PR is pretty raw. You may or may not approve of the approach. I will polish it up after your feedback.