script improvements

- support a user-defined tag for cache entries so that cache-infer
  can remember what it has accomplished if it gets killed or dies

- don't parse all LHSs by default, this was slow and wasn't turning
  up problems

- be verbose, if asked
These options are in my other pull req. Their presence in this script will be hidden and harmless unless someone decides to set the corresponding environment vars, in which case souper will exit with an error.