importfeed: Add feedurl to the metadata

(And allow it to be used in the --template although that seems unlikely to
be very useful.)

My use case for this is that one of the podcast feeds I subscribe to is
sometimes leaking episodes of some other podcast. The other podcast is also
very close to spam, so this may be a form of intentional spamming. I have
not been able to catch the podcast feed containing those episodes, so I
don't know which one is at fault. So putting this in the metadata will let
me eventually catch it.