Integrating the library with an external thread pool

**Is your feature request related to a problem? Please describe.**
The library creates its own thread pool when compressing a large file which then contends with the main application that uses it (especially a problem in i.e. a game engine).

**Describe the solution you'd like**
An API to set a callback for it creating new tasks so you can execute them on your own thread pool when convenient would be nice. Basically fully replacing the pool implementation that comes with the library.

**Describe alternatives you've considered**
Suppose the alternative would be to not use the multi-threading feature within a single task, since we can already run individual compression tasks when convenient.