Add --clean option

[woodb]

From time to time, it would be nice if rural could just clean out everything that's been uploaded to the temporary public bucket.

Maybe something like this...

$ rural --clean
This will delete all files on s3://your-temporary-files-bucket-for-rural, are you sure? [y/N]

By default, --clean should not delete anything (i.e. if you type it and simply hit enter on the "Are you sure?" prompt, it should not delete anything, otherwise delete everything).

If the --verbose or -v flag is used, show all files that are being deleted, otherwise print nothing (save for errors, of course).