config: Fix 'significan' -> 'significant' typo in a comment

Signed-off-by: W. Trevor King <wking@tremily.us>
command: In run(), save feeds even after errors

It's annoying to have a few feeds processed successfully and then have
one feed with a configuration error take down the process without
saving.  With this commit, we always safe the feeds, regardless of any
error.  We also catch and log any RSS2EmailError, not just the
NoToEmailAddress and ProcessingErrors we caught earlier.

Signed-off-by: W. Trevor King <wking@tremily.us>
Bump to version 3.0

Changes since 2.71:
* State storage split into a static configuration file (usually
  `~/.config/rss2email.cfg`) and a dynamic JSON data file (usually
  `~/.local/share/rss2email.json`).
* The static configuration file is parsed with Python's ConfigParser
  class, which allows for default settings that can be overridden on a
  global or per-feed basis.  You'll have to translate your old config
  to the new format by hand when you upgrade.
* Emailed messages now have Message-IDs.
* Feeds can be indexed by name as well as index (e.g.
  `r2e run my-feed`).
* Restructured as a package with submodules instead of a single
  module.  This makes dependencies between various portions of
  rss2email more explicit.
* Converted to Python >=3.2, for more consistent Unicode handling,
  exception chaining, and argparse (although argparse is also in 2.7).
* Packaged with setup.py and distutils, in case you want to install
  rss2email instead of running it from a Git checkout or unpacked
  tarball.
* Added a test suite (run with `./test/test.py`).
* Added a man page, based on the version in the Debian package.

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