Closes #381

This option allows you to change the default imlib2 image cache size of 4 MiB.

might resolve #333 and #348

Sort filelist by dirname, then by name. This results in file entries
sorting before subdirectory entries.

Useful in conjunction with upcoming prev_dir and next_dir navigation
actions.

Purpose of "--no-recursive" option is disabling effect of "--recursive" option that enabled by user's theme.

* Added an --insecure option to disable host/peer verification in libcurl.
This allows feh to load images on HTTPS hosts with self-signed certificates.
For security, this is not the default.

* Fixed an include in show_usage.

The following adds the ability, when using `feh` as a background setter,
to use a specific Xinerama screen to render a provided image. Along with
the provided '--bg-*' option, the '--xinerama-index' argument will be
used to identify the index of the detected Xinerama screen to use.

Signed-off-by: James Knight <james.d.knight@live.com>

this introduces a bug in feh_wm_set_bg, which will be fixed in the next
commit (only applies when run without HOME)

Sort filelist by modification time, newer files first. Useful for
perusing a directory of images by most recently added or changed.

Patch by sdaau on IRC. Thanks!

Problems so far:
* leaks zombie processes
* does not work when terminating feh with a signal (since the convert process
  is no longer in feh's process group)