Add methods for controlling arrow pickup behaviour. Adds BUKKIT-5480

Split from GlowstoneMC/Glowkit#3
Original Bukkit PR: Bukkit/Bukkit#1051
Additional changes: Documentation changes (minor), enum name, tabs to spaces
Glowstone implementation: Not done

-----

Plugin developers may require modifying how an arrow reacts to a player
trying to pick it up (such as ensuring that arrows fired by skeletons can
be picked up normally). This was not possible prior to this commit.