rpm2flatpak: Add an option to initialize from the Flathub manifest
    
    If an application is already packaged on Flathub, allow the user to
    initialize the container.yaml from the Flathub manifest by passing
    
     --flathub=com.example.MyApp
    
    or:
    
     --flathub=MyApp
    
    to do a search (if the search term isn't unique, the operation will
    error out showing a list of matches.)
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>