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

- *_SUPPORT enabled in config.h
- XLIBS helper app prints lib flags needed for current settings

- config.h -> config.def.h
- Create config.h during make, if it does not exist
- Nice make output
- Use XFLAGS and XLIBS to include additional compile-time features,
  which depend on third-party libraries

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