Add Guild#prune (#723)

* make all changes neat

* run docs build

src/structures/Guild.js

@@ -504,6 +504,27 @@ class Guild {
     return create.then(role => role.edit(data));
   }
 
+  /**
+   * Prunes members from the guild based on how long they have been inactive.
+   * @param {number} days Number of days of inactivity required to kick
+   * @param {boolean} [dry=false] If true, will return number of users that will be kicked, without actually doing it
+   * @returns {Promise<number>} The number of members that were/will be kicked
+   * @example
+   * // see how many members will be pruned
+   * guild.prune(12, true)
+   *   .then(pruned => console.log(`This will prune ${pruned} people!`);
+   *   .catch(console.error);
+   * @example
+   * // actually prune the members
+   * guild.prune(12)
+   *   .then(pruned => console.log(`I just pruned ${pruned} people!`);
+   *   .catch(console.error);
+   */
+  prune(days, dry = false) {
+    if (typeof days !== 'number') throw new TypeError('Days must be a number.');
+    return this.client.rest.methods.pruneGuildMembers(this, days, dry);
+  }
+
   /**
    * Causes the Client to leave the guild.
    * @returns {Promise<Guild>}