Ignore list of subdirectories

Recent iOS project was using ShareKit via "git submodule" command, which added a LOT of files into objc_dep graph. Worst of all, I really didn't have any control of those files, just used them as they were given.

To make dependency chart usable again, I added a command line parameter to ignore a list of subdirectories. Tried to keep it compatible with previous "exclude by regex".