Add /ma addreward command.

This command takes an arena player and a Thing as input. The Thing is
parsed in the same way as all other rewards are parsed, so it supports
the new `all()` and `random()` functions and the `nothing` keyword.

The "target audience" of the command is mainly scripts. It opens up the
possibility of hooking into MobArena's rewards from command blocks or
other plugins, but with very low coupling.

Closes #643

changelog.md

@@ -21,6 +21,7 @@ These changes will (most likely) be included in the next version.
 - It is now possible to group rewards. For example, `all(stick, bone)` results a stick and a bone, while `random(all(stick, bone), all(dirt, stone))` results in getting _either_ a stick and a bone _or_ a dirt block and a stone block.
 - The new `nothing` keyword can be used to _not_ grant a reward. This can be used in a crude way to create "loot table"-style reward systems where there is a _chance_ that something is reward, but it might also just be nothing.
 - Boss rewards also support the `all()` and `random()` functions as well as the `nothing` keyword.
+- New command `/ma addreward <player> <thing>` can be used to add a reward to an arena player's reward list. This can be useful for hooking into the rewards system from scripts or other plugins.
 - The Root Target ability now uses potion effects (slowness, slow falling, and negative jump boost) instead of repeated teleports. This should make for a smoother root experience.
 - Using `spectate-on-death: true` no longer forces players out to their join location/exit warp before moving them to the spectator area. This should prevent "jumpy" behavior in multi-world setups.
 - Players should now properly respawn at the spectator area rather than at world spawn on servers with plugins that override respawn locations.