Add a --get parameter to search if there are waivers
This is far from ideal, the current implementation basically does not allow
having multiple actions on the same script, we cannot do things like:
waiverdb-cli new <waivers>
waiverdb-cli get <waivers>
waiverdb-cli delete <waiver>
...
without breaking backward compatibility.
So in order to get the functionality for retrieving waivers, I have added a
--get parameter which allows, using the same syntax to either set or get
waivers against the specified subject.
It's not ideal, but from my testing it does work as expected.
We may consider refactoring this one day, but it is not a priority for the
moment.
Fixes https://pagure.io/waiverdb/issue/152
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>