bob.extension.download should check and be more explicit

This is a set of useful extensions, but:

1. It would be good that the documentation of these functions explicitly state what they are doing: (a) checking the if the directory leading to the destination file exists, otherwise creates it; (b) checks if the file is there - and if so, does not redownload the file.
2. There should be an option to pass an sha256 or md5 checksum to check if the downloaded file is not corrupted. Only if that matches, the file is not re-downloaded
3. There should be an option to "force" re-download no matter what
4. The documentation should also say that the function outputs logs in the info channel.