allow easy data editing via callback

[mknj]

Motivation

I want to change all found elements via a callback. (i.e. rewrite $ref: )
other libraries have an apply(...,callback) method, that allows to change values easily by updating the value with the callback return value.

Current behavior

In this example I want to double all found values.

{ callback: (v,_dummy,p)=>p.parent[p.parentProperty]=v*2 }

This syntax is especially bad if you work with typescript and eslint no_unused_var

Desired behavior

{ applycb: (v)=>v*2 }

[rattrayalex]

An example of how to do this yourself is here: #163 (comment)

Let me know if you have any issues with it…