Enable a diagnostic utility to provide platform details when filing an issue

[asadoughi]

Summary

Instead of users self-reporting the various stats below, let's build a utility method that capture the relevant information:

OS: <!-- e.g. macOS 10.13.3 -->

Faiss version: <!-- git commit, e.g. 56383610bcb982d6591e2e2bea3516cb7723e04a -->

Installed from: <!-- anaconda? compiled by yourself ? --> 

Faiss compilation options: <!-- e.g. using MKL with compile flags ... -->

Running on:
- [ ] CPU
- [ ] GPU

Interface: 
- [ ] C++
- [ ] Python