Add per server prefix (#7)

* Add ability to load server prefix

* Add ability to set prefix for a server

* Create serverPrefixes.json file if not found

commands/prefix.js

@@ -0,0 +1,15 @@
+const setServerPrefix = require("../util/setServerPrefix");
+
+module.exports = {
+    name: 'prefix',
+    description: 'Change the prefix of the bot in this server.',
+    execute({ message, client, args, prefix }) {
+        if (!args[0]) {
+            message.channel.send(`To change the prefix, execute \`${prefix}prefix <newPrefix>\``);
+            return;
+        }else{
+            setServerPrefix(client, args[0], message.guild.id)
+            message.channel.send(`The prefix for this server is now set to ${args[0]}`)
+        }
+    }
+};