config and feed: Added trust-link preference

The trust-link preference allows the user to ignore feed
entries that repeat a previously seen link URL.

Signed-off-by: George Saunders <georgesaunders@gmail.com>
CHANGELOG: Document this branch

And add a warning about toggling the setting for active feeds.

For both George and me, the motivation for this change was working
around feed authors that change the id after minor changes in content:

  On Sat, Jan 18, 2014 at 1:40 PM, W. Trevor King wrote:
  > Some of the newspaper feeds I follow have duplicate entries in
  > their feed if they tweaked the title or content, but I rarely care
  > about the changes.

  On Sat, Jan 18, 2014 at 02:16:19PM -0500, George Saunders wrote:
  > That's exactly the situation I added it for.

The Atom spec explicitly says that revisions should keep the same id
[1]:

  When an Atom Document is relocated, migrated, syndicated,
  republished, exported, or imported, the content of its atom:id
  element MUST NOT change.  Put another way, an atom:id element
  pertains to all instantiations of a particular Atom entry or feed;
  revisions retain the same content in their atom:id elements.

But not all feed generators are fully compliant ;).

[1]: http://tools.ietf.org/search/rfc4287#section-4.2.6

Signed-off-by: W. Trevor King <wking@tremily.us>
Merge branch 'trustlink'

* trustlink:
  CHANGELOG: Document this branch
  config and feed: Added trust-link preference

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