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/Commandlist.java

@@ -116,4 +116,13 @@ public class Commandlist extends EssentialsCommand {
             sender.sendMessage(PlayerList.outputFormat(groupName, PlayerList.listUsers(ess, users, ", ")));
         }
     }
+
+    @Override
+    protected List<String> getTabCompleteOptions(final Server server, final CommandSource sender, final String commandLabel, final String[] args) {
+        if (args.length == 1) {
+            return getGroups();
+        } else {
+            return Collections.emptyList();
+        }
+    }
 }