Add an "-resume" option for the bulk_downloader

[mberacochea]

MGnify has some very large studies, downloading those is problematic. With the current implementation if there is a network issue there is no way to restart the download process using the files already downloaded.

This feature will require (this is just a brain dump)

• Store the tool progress status in a .sqlite db or a text file (pages and the download status for each page, how many pages...)
• Add a "--resume" flag or sniff at the results folder before starting downloading data
• Use the state to start downloading from that point
• Check the files check the downloaded file checksums