mu4e: improve mail-retrieval error handling

Add two new customization variables:
  mu4e-index-update-error-continue
  mu4e-index-update-error-warning

With these, we can configure what happens when the mail-retrieval
program finishes with a non-zero exit code.

Make the default to warn but continue; it seems quite some users got
bitten by the old behavior of not updating after an error (which may
only be a pseudo-error). offlineimap/mbsync do not document their exit
codes very well, unlike fetchmail.

Also update manual for this.
mu4e: add few more header-functions to mu4e-view
fix the target display

show the target when the shown-target is non-nil (not the target).
pass the message structure to the mark actions

This allows to re-tag a message in a mark action.

Two examples of what one can do with this stuff:

1. G-mail archiving:
(archive
   :char       "a"
   :prompt     "archive"
   :show-target (lambda (target) "archive")
   :action      (lambda (docid msg target)
                  (mu4e~proc-move docid nil "-N")
                  (mu4e-action-retag-message msg "-\\Inbox")))

2. Tagging:
(tag
  :char       "g"
  :prompt     "gtag"
  :ask-target (lambda () (read-string "What tag do you want to add?"))
  :show-target (lambda (target) target)
  :action      (lambda (docid msg target)
                 (mu4e~proc-move docid nil "-N")
                 (mu4e-action-retag-message msg (concat "+" target))))
Fixed typo
mu4e: clean up address completion
Merge pull request #525 from jyp/master

mu4e: Allow to do retagging using marks instead of message actions