Implement tab completion for all commands. (#1282)

List of supported commands:
```
/afk
/balance
/balancetop
/ban
/banip
/bigtree
/book
/broadcastworld
/burn
/clearinventory
/condense
/delhome
/deljail
/delwarp
/eco
/enchant
/enderchest
/essentials
/exp
/ext
/feed
/fireball
/firework
/gamemode
/getpos
/give
/hat
/heal
/help
/helpop
/home
/ignore
/invsee
/item
/itemdb
/jump
/kick
/kill
/kit
/lightning
/list
/mail
/me
/msg
/mute
/near
/nick
/nuke
/pay
/potion
/powertool
/ptime
/pweather
/recipe
/remove
/repair
/sell
/showkit
/skull
/speed
/tempban
/thunder
/time
/togglejail
/tp
/tpa
/tpaall
/tpahere
/tpall
/tphere
/tpo
/tpohere
/tppos
/tree
/warp
/weather
/world
/worth```

Essentials/src/com/earth2me/essentials/commands/Commandweather.java

@@ -2,9 +2,13 @@ package com.earth2me.essentials.commands;
 
 import com.earth2me.essentials.CommandSource;
 import com.earth2me.essentials.User;
+import com.google.common.collect.Lists;
 import org.bukkit.Server;
 import org.bukkit.World;
 
+import java.util.Collections;
+import java.util.List;
+
 import static com.earth2me.essentials.I18n.tl;
 
 
@@ -62,4 +66,32 @@ public class Commandweather extends EssentialsCommand {
             sender.sendMessage(isStorm ? tl("weatherStorm", world.getName()) : tl("weatherSun", world.getName()));
         }
     }
+
+    @Override
+    protected List<String> getTabCompleteOptions(Server server, User user, String commandLabel, String[] args) {
+        if (args.length == 1) {
+            return Lists.newArrayList("storm", "sun");
+        } else if (args.length == 2) {
+            return COMMON_DURATIONS;
+        } else {
+            return Collections.emptyList();
+        }
+    }
+
+    @Override
+    protected List<String> getTabCompleteOptions(Server server, CommandSource sender, String commandLabel, String[] args) {
+        if (args.length == 1) {
+            List<String> worlds = Lists.newArrayList();
+            for (World world : server.getWorlds()) {
+                worlds.add(world.getName());
+            }
+            return worlds;
+        } else if (args.length == 2) {
+            return Lists.newArrayList("storm", "sun");
+        } else if (args.length == 3) {
+            return COMMON_DURATIONS;
+        } else {
+            return Collections.emptyList();
+        }
+    }
 }