Feature request: progress indicator

[danestarija]

It would be nice if users could provide a reference of/pointer to atomic int/float.
This would enable users to check how many bytes have been written while the target object is being serialized.

I am/was trying to serialize a vector<tuple<short,short,short,short,short,short>> with around 50000 elements
I think it would be nice to know how far along am I with this serialization by reading into an atomic int/float via another thread and printing it to stdout.