changes to tab completion

* PacketCBTabComplete: options for completion should be seperated by
NUL
 * PluginManager: append an empty argument to arguments if command ends
with a whitespace (this will match all suggestions)
 * PlayerCommand: suggest only matching players instead of all players

api/src/main/java/net/md_5/bungee/api/plugin/PluginManager.java

@@ -137,7 +137,18 @@ public class PluginManager
             return true;
         }
 
-        String[] args = Arrays.copyOfRange( split, 1, split.length );
+        String[] args;
+
+        // add empty argument for tab completion
+        if ( commandLine.endsWith( " " ) )
+        {
+            args = Arrays.copyOfRange( split, 1, split.length + 1 );
+            args[ split.length - 1] = "";
+        } else
+        {
+            args = Arrays.copyOfRange( split, 1, split.length );
+        }
+
         try
         {
             if ( tabResults == null )