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.)