feat(RoleStore, ChannelStore): fetch() method (#3071)

* feat({Role,Channel}Store): fetch method

* docs: Add usage examples to the new methods

* misc: Add note of why we are fetching all roles even for a single one

src/stores/ChannelStore.js

@@ -74,6 +74,24 @@ class ChannelStore extends DataStore {
     super.remove(id);
   }
 
+  /**
+   * Obtains a channel from Discord, or the channel cache if it's already available.
+   * @param {Snowflake} id ID of the channel
+   * @param {boolean} [cache=true] Whether to cache the new channel object if it isn't already
+   * @returns {Promise<Channel>}
+   * @example
+   * // Fetch a channel by its id
+   * client.channels.fetch('222109930545610754')
+   *   .then(channel => console.log(channel.name))
+   *   .catch(console.error);
+   */
+  fetch(id, cache = true) {
+    const existing = this.get(id);
+    if (existing) return Promise.resolve(existing);
+
+    return this.client.api.channels(id).get().then(data => this.add(data, null, cache));
+  }
+
   /**
    * Data that can be resolved to give a Channel object. This can be:
    * * A Channel object