Adds a highly configurable /heal command used to heal players.

The mod is configured via gamerules:

• healAmount sets how much each /heal restores, defaulting to 200 (100 hearts).
• healCost sets how much health each /heal removes from the healer, defaulting to 0.
• healsPerLife sets how many uses of /heal each player has, defaulting to 1 (set to -1 for infinite uses).
• deathsRestoringHeals controls now many deaths will restore healing capacity, defaulting to -1 (set to -1 for unlimited deaths).
• healPermissionsRequirement is what permissions level is needed to run /heal, defaulting to 0 (anyone can use it).
• safeHealing controls, given that there is a healCost, whether or not players can fatally damage themselves by /healing other players. Defaults to true; players are prevented from using /heal when they have less health than is needed to pay the cost.

The /heal command itself uses a relatively simple syntax with several options at the moment. /heal <player> heals the target by the current healAmount and damages the healer by healCost. It is available to any player with permissions higher than healPermissionsRequirement. /heal query uses <player> displays the number of /heal uses that the target has expended, as well as the current value of healsPerLife. It is available to any player with a permission level of at least 2. /heal query deaths <player> displays the number of deaths that the target has remaining that will restore healing capacity. It is available to any player with a permission level of at least 2. /heal restore uses <player> restores all /heal uses to the target. It is available to any player with a permission level of at least 2. /heal restore uses <player> n restores n uses of /healto the target; this can exceed healsPerLife total uses, giving that player extra capacity. It is available to any player with a permission level of at least 2. /heal restore deaths <player> resets the deaths counter for the target. It is available to any player with a permission level of at least 2. /heal restore deaths <player> n restores n deaths to the target; this can exceed deathsRestoringHeals total deaths, giving that player extra capacity. It is available to any player with a permission level of at least 2.