Misc #136: Add CPPFLAGS support

Thanks to smlx for reporting.
Improvement #137: --xinerama-shadow-crop

Add --xinerama-shadow-crop to crop shadow to current Xinerama screen.
Thanks to Feltzer for suggestion.
Misc: Add missing configuration file options and switches

- Add "xinerama_shadow_crop" configuration option.

- Add long commandline switches corresponding to the short ones.

- Documentation update.
Bug fix #137: Auto-detect screen changes

Attempt to auto-detect screen changes to address the issue reported by
Feltzer.