New API: visitor override?

[barneycarroll]

I'm iterating over a recursive structure where it's undesirable for undefined values in the patch to replace their counterparts in the target.

According to the literal declarative nature of Patchinko this would be an intuitive scenario to create a new iteration token case for (I for ignore?) but in practice this is achievable with S by simply returning the input.

This proposal is for a function that can be supplied to P and run after patch iteration with the following arguments: key, source, patch, replace, replace being a function that can be invoked to replace the patch value.

Thus the generic 'skip nullish' scenario would read as:

O(a,b, ({source, patch, replace}) => (
  patch == null 
? 
  replace(source)
: 
  patch
)

[kristianmandrup]

Looks awesome! Why don't you make a PR with a test demonstrating it in practice and some docs :)