V2.0 - multithreading

I've rewritten afetch to use the pthreads API, and have tested it on MacOS, FreeBSD, OpenBSD and Alpine Linux. It seems to be slightly faster on some operating systems, and much faster on others.

I'll have to do some more optimisation soon, as a lot of the code that came from V1.0 is very inefficient. I'm also not able to test every Linux distro, so please tell me if somethings not working as expected. Thanks <3