dirty code, POC.

@Twitter.java

package com.colt.TestPlugin;

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.ProtocolException;
import java.net.URL;

import net.md_5.bungee.api.ChatColor;
import net.md_5.bungee.api.chat.ClickEvent;
import net.md_5.bungee.api.chat.TextComponent;

import org.bukkit.Bukkit;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.AsyncPlayerChatEvent;
import org.bukkit.plugin.java.JavaPlugin;

public class TestPlugin extends JavaPlugin implements Listener { 
	
	public void onEnable() {
		Bukkit.getServer().getPluginManager().registerEvents(this, this);
	}
	
	@EventHandler
	public void chat(AsyncPlayerChatEvent e) {
		String pattern = "@\\w{1,15}";
		String msg = e.getMessage();
		if(msg.contains(pattern) && isValidTwitter(msg) == true) {
			TextComponent ee = new TextComponent(msg);
			ee.setColor(ChatColor.RED);
			ee.setClickEvent(new ClickEvent(ClickEvent.Action.OPEN_URL, "https://www.twitter.com/" + msg.replace("@", "")));
			e.getPlayer().spigot().sendMessage(ee);
			e.setCancelled(true);
		}
	}	
	
	public static boolean isValidTwitter(String username) {
		try {
			URL u = new URL("https://www.twitter.com/" + username.replace("@", ""));
			HttpURLConnection huc = (HttpURLConnection)u.openConnection(); 
			huc.setRequestMethod("HEAD");
			huc.connect(); 
			if(huc.getResponseCode() == 404) {
				return false;	
			} else if(huc.getResponseCode() == 200) {
				return true;
			}
		} catch (MalformedURLException e) {
			e.printStackTrace();
		} catch (ProtocolException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return false;
	}
}