GitHub powered ranking

This commit improves the search ranking algorithm embedding a popularity in each library. It is based on the number of GitHub stars.

Basically, before pushing the libraries to Algolia's REST API for indexing, the script calls the GitHub API to fetch the associated stargazer count, fork count, ...

The good thing with popularity is that it now retrieves relevant results from the first keystroke:
![github powered ranking](http://g.recordit.co/vJRAIjeSsh.gif)

I've already re-indexed the libraries with their associated popularity using my local branch so you can test the relevance with the current UI.

The API is rate-limited to 5,000 queries per hour so I think it will not be a problem here.

Let me know what you think!