Uses JSON::Feed, for the moment, although it might be worth just rolling
my own to avoid the dependency.

Adds:

  - feed_print_json_recent()
  - feed_print_json()

Also puts a decode() call around feed_print() output and adds open()
pragmas to a couple of utilities.

Uses Term::ReadLine, but I think I probably need a different backend (Perl
or GNU ReadLine implementations) for it to really do much.