Add a fallback mapping for the shared libraries, providing support for (at least) OSX darwin

Hey I really like this library from what I've seen so far, especially that it's easy to embed in Jupyter notebooks.

In order to get it to work on my mac (OSX darwin), I had to make a couple changes to how modules are loaded (you'd be surprised how many python projects can be made to work on mac using ctypes.util.find_library). After this change I was able to run everything in qahirah_notebooks without issue.

Let me know what you think of the merge request, it may be that find_library() also works well for linux but I thought I'd leave the rest of your code as it is since I don't have a linux (or bsd) system to test it on right now.