Compare package download counts

Hi,
It could be very interesting to add a "**Compare package download counts over time**" like [npmtrends.com](https://www.npmtrends.com/)

Actually we have only install statistics for a package, but we can have a comparator also like **Symfony vs Laravel vs Zend**...

Start by reviewing the existing package install-statistics implementation and the npmtrends.com reference linked in the issue. Determine how package histories are collected and displayed, then define the comparison scope and acceptance criteria for examples such as Symfony, Laravel, and Zend. Done means users can compare package download counts over time.

Written by the indexing model from the issue text.