0.7.0: support for progress callback in `Downloader`

• f027c64 doc: various improvements in the doc
• 6a169e4 refactor!: hide the private types used to build the Downloader
• 8fc1247 doc: improved documentation of Downloader
• 4d0078c feat: added Downloader::on_progress
• bb662e4 refactor: prepare Downloader for on_progress callback
• 7290846 doc: changed the URL in the example to use the samples from github
• 7d43c82 cleanup: only import tokio with macros feature enabled
• 1512dab chore: prepare for next version