Added flag to 'spec' to only clear spec files

- The new flag will only clear spec files from
  the require and global caches when 'tishadow
  spec' is run.

- If the new flag is not specified then the
  entire cache is cleared as normal (current
  behavior).
Merge pull request #289 from sztn/clear_spec_files

Added flag to 'spec' to only clear spec files