Support multi-threaded downloads

[Richienb]

This would be useful when downloading large files and is already a strategy used by Chrome in which it creates .crdownload files.

Prior art: https://github.com/FaisalUmair/mt-files-downloader

// @sindresorhus

[Richienb]

I made an implementation of multipart downloading that we can base changes off: https://github.com/Richienb/multipart-get