Add stop command alias for ShutdownCommand

Fixes #678

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -209,7 +209,8 @@ public class VelocityServer implements ProxyServer, ForwardingAudience {
     // Initialize commands first
     commandManager.register("velocity", new VelocityCommand(this));
     commandManager.register("server", new ServerCommand(this));
-    commandManager.register("shutdown", ShutdownCommand.command(this), "end");
+    commandManager.register("shutdown", ShutdownCommand.command(this),
+        "end", "stop");
     new GlistCommand(this).register();
 
     this.doStartupConfigLoad();