importfeed: Support -J (and work toward supporting --json)

Both -J and --json needed importfeed to be refactored to use commandAction.

That was difficult, because of the interrelated nature of downloading feeds
and then downloading files from feeds, both of which needed to use
commandAction. And then checking for problems in feeds has to come after
these actions, which may be run as background jobs.

As for --json support, it's most of the way there, but still has some
warts, so I didn't enable jsonOptions yet. The warts include:

- An initial empty json record is displayed by getCache.
- Input is not populated, should be feed url
- feedProblem at end will not be captured by --json-error-messages
  (see FIXME)

Sponsored-By: the NIH-funded NICEMAN (ReproNim TR&D3) project

doc/todo/importfeed_parallell.mdwn

@@ -1,3 +1,5 @@
 Make git-annex importfeed support -J. Ideally, when run with multiple feed
 utls, it would check them in parallel, and also parallelize download of new
 items found in the feeds. --[[Joey]]
+
+> [[done]] --[[Joey]]