Added flag to 'spec' to only clear spec files

We started using TiShadow for test automation in our applications.  We wanted a mechanism to rerun specs on a running appified application but not clear the entire require and global caches each time.  So a new flag was added to 'tishadow spec' to only clear spec files from the caches.  If the flag is not specified then 'tishadow spec' behaves as it currently does and clears the entire cache.

I went ahead and updated the README.md file to reflect the new flag, not sure if you wanted this so I can happily remove it if necessary.