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.

Command/ImportFeed.hs

@@ -537,7 +537,8 @@ minimalMetaData i = case getItemId (item i) of
  - and to generate the filename. -}
 extractFields :: ToDownload -> [(String, String)]
 extractFields i = map (uncurry extractField)
-	[ ("feedtitle", [feedtitle])
+	[ ("feedurl", [Just (feedurl i)])
+	, ("feedtitle", [feedtitle])
 	, ("itemtitle", [itemtitle])
 	, ("feedauthor", [feedauthor])
 	, ("itemauthor", [itemauthor])