command: Fix opmlexport crash due to orphaned feed data

command: Fix opmlexport crash due to orphaned feed data

When you remove a feed from your config file by hand, you might leave
the dynamic 'seen' data in the JSON data file by accident.  If you
have such orphan data, the feed is loaded by Feeds._load_feeds() with
the default configuration (since you removed the config file entry).
This can lead to opmlexport errors like:

  Traceback (most recent call last):
    File "./r2e", line 5, in <module>
      rss2email.main.run()
    File "/.../rss2email/rss2email/main.py", line 163, in run
      args.func(feeds=feeds, args=args)
    File "/.../rss2email/rss2email/command.py", line 157, in opmlexport
      url = _saxutils.escape(feed.url)
    File "/usr/lib64/python3.2/xml/sax/saxutils.py", line 34, in escape
      data = data.replace("&", "&amp;")
  AttributeError: 'NoneType' object has no attribute 'replace'

because the feeds lack the per-feed 'url' setting that had been
defined in the config file.  With this commit, opmlexport drops these
URL-less feeds, instead of choking to death trying to format them ;).

Signed-off-by: W. Trevor King <wking@tremily.us>