Suppress "Downloading" message in download_file()

This is brought up in #552 by @taldcroft, "There should definitely be a way to make this code run with absolutely no output. If I have an application that calls the cone search function I want complete control over the screen output. If it's not already possible to suppress output in download_file() then there should be a feature request put in."

This is a feature request for the ability to silence `astropy.utils.data.download_file()`.

https://github.com/astropy/astropy/blob/3cf73b574caca1fd375ae29121f1e51dd65785b6/astropy/utils/data.py#L782