mu4e: add mu4e-headers-split-view-shrink
mu4e: add mu4e-scroll-{up,down}, use {beginning,end}-of-buffer
mu4e-sent-messages-behavior: fix custom type defintion
mu4e-maildir-shortcuts: define custom type
mu4e-view-mode-hook: define explicitly

Explicit offer `turn-on-visual-line-mode' as hook function.
* make sure mu4e-compose-parent-message is available in mu4e-compose-mode as
  well (thanks to Tsunenobu Kai)
* cosmetic
* some fixes for the retagging action (thanks to Abdó Roig-Maranges)
Merge pull request #113 from tarsius/custom

* various improvements for 'defcustom' usage and explicitly offer `turn-on-visual-line-mode' as hook function.
* mu-msg-iter: add basic implemenation of checking for msgid dups, msg file
  existence
* mu-query: update for mu-msg-iter updates
* cosmetic
* mu-msg-iter.{cc,h}: make flag names a bit clearer
* add query flags (which match msgiterflags) to mu_query_run, update callers
* add --skip-msgid-dups / -u option to mu find, document it
* fix `mu4e~headers-human-date'
* add support for searching the mailing list of a message
* cosmetics
* configure: bump version to v5
* add basic support for skipping dups, unreadable messages in mu-msg-iter