feed: Split entry link extraction out into Feed._get_entry_link

Now other methods can all access the same link, without having to
extract it in _process_entry and pass the extracted link around
explicitly.  Currently, nothing special happens during link
extraction, but the new method helps future proof us.

Signed-off-by: W. Trevor King <wking@tremily.us>
feeds: Use JSON instead of Pickle for storing dynamic feed state

It's safer, more portable, and possibly faster.  I've also added
version information to the data file for easier future upgrades.

Signed-off-by: W. Trevor King <wking@tremily.us>
feeds: Follow the the XDG Base Directory Specification

This splits config files and data files into different directories (by
default), so we no longer need an rss2email subdirectory (we only have
one config file and one data file).  The default config file is now
~/.config/rss2email.cfg and the default data file is now
~/.local/share/rss2email.json.

Signed-off-by: W. Trevor King <wking@tremily.us>
r2e.1: Update maintainer from Lindsey to me.

This should have happened in:

  commit 6460e8738b5e7c66df6c2143e8a29c048cf308bd
  Author: W. Trevor King <wking@tremily.us>
  Date:   Fri Nov 9 07:46:17 2012 -0500

    Change maintainer from Lindsey to Trevor.

Signed-off-by: W. Trevor King <wking@tremily.us>
r2e.1: Update __contributors__ location and direct bugs to the mailing list

Signed-off-by: W. Trevor King <wking@tremily.us>
r2e.1: Update ConfigParser URL after PEP 430

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