Feature: allow queue callback function parameter

[reabreu]

Hello and thanks for your work! I've added an optional callback parameter to the append function cause we needed it to handle backpressure on an intense export feature we're developing. Please have a look and let me know if we can add it to the library.

[reabreu]

Addresses #470 #414 #398 #385

[manglass]

Any word on this being merged?

[reabreu]

Hey, no word yet.