Stable version 1.1.1
Fixed typo in README.md
Handle 180 degrees image rotation
Refactored merged rotation code
Changed key bindings for flipping to bar & underscore
Updated unstable version number
Added file marks; fixes issue #94

- 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
Only print marked files when -o is given

Fallback to print all files, if no images are marked, was kind of
irritating.
Added options for anti-alias & alpha layer coloring to config.def.h

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