mu4e: add mu4e-headers-find-if-next, mu4e-headers-next-unread

In the headers-view, allow for movig to next and previous
unread/untrashed messages using tab/backtab.

Built on top of the convenience function mu4e-headers-find-if-next.

Update docstrings.
Put the definitions of marks in a single structure

This commit puts all what defines a mark (how to display, how to handle
targets, what action to apply, ...) in a single entry of a list. The list
is stored in a variable.
This should allow for customization of marks.
Merge pull request #523 from jyp/master

mu4e: put the definitions of marks in a single structure
configure.ac: bump version to 0.9.11
mu4e: update documentation
always recompute the dynamic target for a mark

This is a bug fix. Previously, recomputing was done only for refile,
which is wrong: trash target can also be dynamic, and we want to allow
the user to configure more dynamic targets.
documentation fix
mu4e: marks: clarify the way targets are obtained.

:get-target is split into:
- :ask-target (run once per bulk operation)
- :dyn-target (run once per message)

A side benefit is that the existence of the target of a move is not
checked for every message when doing bulk moves.
Merge pull request #524 from jyp/master

Clarify the computation of mark targets
mu4e: support jumping to next, previous unread message

mu4e-headers-next-unread
mu4e-view-headers-next-unread

Use key-bindings ] and [ to go to next, previous unread message.
Merge branch 'master' of github.com:djcb/mu
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
mu4e: Make :show-target be the identity by default

- This is certainly a better default than crashing
- The user is likely to omit this function in a majority of cases.
take a stab at the documentation of mu4e-marks