None of the mouse mappings uses a keyboard modifier, making it possible to
access the most basic features by only using the mouse.

Next/previous image with left button depending on cursor position, middle
button for dragging, right button for switching to thumnail mode and wheel for
zooming.

Users can keep the old behaviour by simply not adapting the changes to the
buttons array in config.def.h to their config.h file.

- Key mappings +/- are now general commands
- Use JPG as thumbnail cache file format instead of PNG
- Fixes issue #161

- New option `-a`: Play animations at startup
- Ctrl-Space toggles animation for all GIF files
- Infinite loop for all animations

Fixes issue #143

- Scale mode is not reset to default value upon image loading anymore
- New default key binding to change mode to scale-down
- Removed scale mode setting from config.h
- Removed -d command line option, as this is now the default at startup

The default prefix key is C-x, and can be changed in config.def.h. The
first key pressed after the prefix key will be passed the external key
handler, unless the key is Escape, which is used to cancel the prefix.

Gets called on all unset key mappings. Arguments are: key combo and
current file. Thanks to Francesco Orsenigo (xarvh) for the idea.

Also removed now obsolete -p command line option; fixes issue #98

- Command it_toggle_image_mark (bound to 'm') toggles mark of current
  image
- Command it_navigate_marked (bound to 'N'/'P') can be used to go to
  the next/previous marked image
- When option -o is given, all marked files get printed