Feature: Build feature to automatically download arrayfire binaries

Description
===========
Hi there! I'm thinking about using arrayfire-rust (through [mushin](https://github.com/c0dearm/mushin)) and will get around to trying it out sometime, but I noticed the need to manually download and install the AF binaries. This is not that much of an issue (and won't stop me from using it), but as a matter of convenience, is it possible to offer a feature that will automatically download the AF binaries as part of the build script?

There is precedent for this (e.g. here's [an old example](https://github.com/maidsafe/rust_sodium/blob/master/rust_sodium-sys/build.rs)), but as it'd make the build non-isolated / potentially unreproducible, it should be an optional feature.

The ideal solution is still probably #192, but this should be a reasonable middle ground if users don't mind the downstream application downloading/bundling copies of AF.