invalidations instead of only the ones that hit the cache.

- Support reading gzipped files.  This is triggered automatically when
  the filename ends in .gz.  Also support reading from stdin (pass '-'
  as filename).

- Widen the FLIPS column by one to match the EVICTS column width of
  the LRU simulation.

- Support simulating alternative caching strategies.  For now, only
  LRU is supported, triggered by the -l option.

- Change the Simulation base class to store the cachelimit argument.
  (The constructor signature must be the same across all subclasses.)