- 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

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

Fallback to print all files, if no images are marked, was kind of
irritating.

- 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

Fixes issue #84

- Configurable key and mouse mappings in config.h
- Put event handling code from main.c into events.[ch]