Add a way to prevent Slimes from Spawning

Now that Caves and Cliffs is out I have found that building underground can be quite frustrating because we have no ability to control or prevent Slimes from spawning. Making them only spawn in darkness would be a problem because I’m sure many players have made Slime farms that would be ruined if the lighting rules suddenly applied to them. I suggest adding something like a Salt Block that would make the area around them too dry for Slimes to spawn. Maybe they could also be harmful to Slimes if they make contact. Salt could be mined as a new ore or found in a new biome like the Salt Flats.