* nnmairix.el: Upgrade to version 0.6. FIXME: Add entry on

* nnmairix.el: Upgrade to version 0.6.  FIXME: Add entry on
function/variable level.

- It is now possible to propagate marks from the nnmairix groups
  to the original messages (and for maildir also vice versa). See
  the docs for details on this feature - it's pretty delicate
  and currently needs a patched mairix binary to work smoothly.

- Keep messages in nnmairix groups always read/unread
  (bound to `G b r').

- Recreate back end folder for nnmairix groups in case you
  somehow get wrong article counts (bound to `G b d').

- New group parameter `allow-fast'. Toggling of parameter bound to `G b
  a'.  The default is nil, meaning that the group will always be
  updated with a mairix search, even when only entered.

- More/Better use of the registry (if available).  Can now also deal
  with duplicate messages in different groups.