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

util/loadServerPrefixes.js

@@ -0,0 +1,18 @@
+const fs = require('fs')
+
+
+module.exports = function (client) {
+    try {
+        if(!fs.existsSync('serverPrefixes.json')){
+            console.log("Creating loadServerPrefixes.json...")
+            fs.writeFileSync("serverPrefixes.json","[]")
+        }
+        const json = fs.readFileSync('serverPrefixes.json', 'utf8');
+        const serverPrefixes = JSON.parse(json);
+        serverPrefixes.forEach(server => {
+            client.serverPrefixes.set(server.id, server.prefix)
+        });
+    } catch (err) {
+        console.log(`Error reading file from disk: ${err}`);
+    }
+}