Add optional support for libnotify notifications.

Added suport for [libnotify](https://developer.gnome.org/libnotify/) as a compile-time dependency with the WITH_LIBNOTIFY flag, enabled by default. When not enabled, any use of the function will result in a girara style notification to the user explaining him that girara was compiled without it.

The notification will try to use the icon in the following order:
 - If there is a "named icon" such as the ones in /usr/share/icons, it will try
   to pick it up.
 - If there is an icon file, it will try to pick it up.
 - If there is nothing, it will use the "info" stock icon.

Refer to the "window-icon" and it associated callback for more information on icon handling.
If this call fails and error is set, memory will be leaked. If you want to use error for something, use it here and free it. Otherwise just pass NULL and get rid of error.