[FEATURE] Provide download information by day/month/year

Currently it's possible to get the downloads by version with `https://packagist.org/packages/[vendor]/[package]/downloads.json`.

The problem is that if the day / month / year is over, you can't get the number of downloads of the last day / month / year.

Would be nice to have the download numbers by a parameter which gives you a year (`2018`), a month (`2019-01`) or a day (`2019-01-22`).

Start by examining the existing downloads.json endpoint and how its version-based download data is produced. Determine how year, month, and day parameters should retrieve completed periods, then verify that each requested period returns the expected download count.

Written by the indexing model from the issue text.