Added prefix command

commands/prefix.js

+const Discord = require('discord.js');
+exports.run = async(bot, message, args, connection) => {
+
+    let prefix = args[0];
+    let guildid= message.guild.id;
+    var sql = `UPDATE guilds SET prefix = '`+ prefix +`' WHERE guildid = '`+ guildid +`'`;
+    connection.query(sql, function (err, result) {
+      if (err) throw err;
+      message.channel.send("prefix changed succesffully to "+ prefix);
+    });
+      
+};
+
+module.exports.help = {
+    name: 'prefix',
+    aliases: ['setprefix']
+};