HyperRogue bugfix

Sorry for those who have found crashes when playing with mouse and mirrors, or Ivy plants not challenging enough. Version 2.1, still in time for the 7DRLC, has the following bugfixes and minor additions:

• The game should no longer crash when moving in certain directions while having mirror images.
  
• Living Caves now include Seeps and are thus now more challenging.
  
• You can no longer deactivate an ivy plant by cutting out an active branch next to the root.
  
• Shield protection from a sandworm is now less weird.
  
• Slime colors are now named correctly.
  
• Some sanity checks added just in case.
  

Thanks to Eelfroth (on 7drl.org) for reporting the crash bug, and the slime color bug. Download from the HyperRogue website. Have fun!