Update to 2.18.1

Changes from https://git.finalrewind.org/feh/plain/ChangeLog
Sun, 22 Jan 2017 19:11:32 +0100  Daniel Friesel <derf+feh@finalrewind.org>

* Release v2.18.1
    * Fix image-specific format specifiers not being updated correctly
      (e.g. %z not displaying the correct zoom value after zooming in / out)

Tue, 01 Nov 2016 10:55:04 +0100  Daniel Friesel <derf+feh@finalrewind.org>

* Release v2.18
    * Move README to README.md
    * New key binding: ! / zoom_fill (zoom to fill window, may cut off image
      parts)
    * Only for builds with exif=1: Disable EXIF-based auto rotation by
      default, add --auto-rotate option to enable it (Patch by Elliot Wolk)

Wed, 31 Aug 2016 20:27:20 +0200  Daniel Friesel <derf+feh@finalrewind.org>

* Release v2.17.1
    * Fix compilation on systems where HOST_NAME_MAX is not defined, such as
      FreeBSD (patch by Niclas Zeising)

Sun, 28 Aug 2016 21:26:54 +0200  Daniel Friesel <derf+feh@finalrewind.org>

* Release v2.17
    * Install feh icon (both 48x48 and scalable SVG) to /usr/share/icons
      when running "make install app=1"
    * Fix --sort not being respected after the first reload when used in
      conjunction with --reload
    * All key actions can now also be bound to a button by specifying them
      in .config/feh/buttons. However, note that button actions can not be
      bound to keys.
    * Rename "menu" key action to "toggle_menu", "prev" to "prev_img" and
      "next" to "next_img". The old names are still supported, but no longer
      documented.
    * feh now also sets the X11 _NET_WM_PID and WM_CLIENT_MACHINE window
      properties

Sun, 31 Jul 2016 16:59:07 +0200  Daniel Friesel <derf+feh@finalrewind.org>

* Release v2.16.2
    * Also support in-place editing for images loaded via libcurl or
      imagemagick. Results will not be written back to disk in this case.

(szptvlfn)