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/Commandpotion.java

@@ -1,5 +1,8 @@
 package com.earth2me.essentials.commands;
 
+import org.bukkit.DyeColor;
+
+import com.google.common.collect.Lists;
 import com.earth2me.essentials.MetaItemStack;
 import com.earth2me.essentials.Potions;
 import com.earth2me.essentials.User;
@@ -11,6 +14,8 @@ import org.bukkit.inventory.meta.PotionMeta;
 import org.bukkit.potion.PotionEffect;
 import org.bukkit.potion.PotionEffectType;
 
+import java.util.Collections;
+import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
@@ -70,4 +75,35 @@ public class Commandpotion extends EssentialsCommand {
             throw new Exception(tl("holdPotion"));
         }
     }
+
+    @Override
+    protected List<String> getTabCompleteOptions(Server server, User user, String commandLabel, String[] args) {
+        // Note: this enforces an order of effect power duration splash, which the actual command doesn't have.  But that's fine. 
+        if (args.length == 1) {
+            List<String> options = Lists.newArrayList();
+            options.add("clear");
+            if (user.isAuthorized("essentials.potion.apply")) {
+                options.add("apply");
+            }
+            for (Map.Entry<String, PotionEffectType> entry : Potions.entrySet()) {
+                final String potionName = entry.getValue().getName().toLowerCase(Locale.ENGLISH);
+                if (user.isAuthorized("essentials.potion." + potionName)) {
+                    options.add("effect:" + entry.getKey());
+                }
+            }
+            return options;
+        } else if (args.length == 2 && args[0].startsWith("effect:")) {
+            return Lists.newArrayList("power:1", "power:2", "power:3", "power:4");
+        } else if (args.length == 3 && args[0].startsWith("effect:")) {
+            List<String> options = Lists.newArrayList();
+            for (String duration : COMMON_DURATIONS) {
+                options.add("duration:" + duration);
+            }
+            return options;
+        } else if (args.length == 4 && args[0].startsWith("effect:")) {
+            return Lists.newArrayList("splash:true", "splash:false");
+        } else {
+            return Collections.emptyList();
+        }
+    }
 }