Add --offline cli argument for new-sources
Cli command new-sources calls command upload,
which already had an offline mode.
Only thing needed for new-sources --offline support
was to add the argument and pass it to upload command.
Relates: #559
Merges: https://pagure.io/rpkg/pull-request/561
Signed-off-by: Otto Urpelainen <oturpe@iki.fi>