Added cleanContent getter (#644)

* Added cleanContent getter which replaces mentions with the equivalent
text.

(Majority of code implemented by Gus).

* Fixed bug where invalid mentions would cause crash

* Fixed not cleaning @everyone and @here

* Fixed only detecting first @here or @everyone, resolved to test software (and learn regular expressions) in the future

src/structures/Message.js

@@ -155,6 +155,43 @@ class Message {
   }
 
   /**
+   * The message contents with all mentions replaced by the equivalent text.
+   * @type {string}
+   */
+  get cleanContent() {
+    return this.content.replace(/<@!?[0-9]+>/g, input => {
+      let user = this.channel.guild.members.get(input.replace(/<|!|>|@/g, ''));
+
+      if (user) {
+        if (user.nickname) {
+          return `@${user.nickname}`;
+        }
+        return `@${user.user.username}`;
+      }
+
+      return input;
+    }).replace(/<#[0-9]+>/g, (input) => {
+      let channel = this.client.channels.get(input.replace(/<|#|>/g, ''));
+
+      if (channel) {
+        return `#${channel.name}`;
+      }
+
+      return input;
+    }).replace(/<@&[0-9]+>/g, (input) => {
+      let role = this.guild.roles.get(input.replace(/<|@|>|&/g, ''));
+
+      if (role) {
+        return `@${role.name}`;
+      }
+
+      return input;
+    })
+    .replace(/@everyone/g, '@\u200Beveryone')
+    .replace(/@here/g, '@\u200Bhere');
+  }
+
+ /**
    * An array of cached versions of the message, including the current version.
    * Sorted from latest (first) to oldest (last).
    * @type {Message[]}