Add option to save files by default instead of printing to stdout.

[andreipoe]

Hello again,

As I mentioned in #6, I have made another change to the script, motivated by the fact that I expect the default behaviour to save the downloaded file instead of printing it to stdout. So, I've added a configuration option at the top of the script to do just that. However, I understand that one of your initial goals when making this was to be pipe-friendly, and this change may affect that, so I've put it in a separate pull request.

Note that I haven't changed the default way the script works, but setting use_stdout=no will cause it to save downloaded file by default, as well as print the upload URL on a separate line.