Suggestion: continue download

[martindurant]

When downloading large files, it can commonly happen that the transfer aborts part way. Rather than having to restart the process, it would be nice to be able to read only the remaining bytes on a subsequent attempts. cat_file() already allows for providing a start byte (and end byte), but get_file() does not. I am proposing to both implement start/end byte and, more importantly, to allow "continue" whereby start=size-of-local-file.