Closes: #124

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

* 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.

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!

It is pretty random and I doubt anyone actually uses it. Also, after removing
it, there are only two duplicates of the image index code instead of three...