Add client.getMessage() (#428)

* Add client.getMessage

* Add shortcuts

* build it..

* Add missing ~ in note docs

* ............

src/Client/Client.js

@@ -611,6 +611,34 @@ export default class Client extends EventEmitter {
 			.then(dataCallback(callback), errorCallback(callback));
 	}
 
+	/**
+	 * Gets a single message of a server
+	 * @param {ChannelResolvable} channel to get the message from
+	 * @param {function(err: Error, msg: Message} [callback] callback to the method
+	 * @returns {Promise<Message, Error>} Resolves with a message if the request was successful, otherwise rejects with an error.
+	 * @example
+	 * // get message object off a snowflake and log its content - callback
+	 * client.getMessage(channel, '192696158886428672', function(err, msg) {
+	 *     if(!err) {
+	 *         console.log(msg.content);
+	 *     } else {
+	 *         console.log("couldn't get the message");
+	 *     }
+	 * }
+	 * @example
+	 * //get message object off a snowflake and log its content - promise
+	 * client.getMessage(channel, '192696158886428672')
+	 *     .then(msg => {
+	 *         console.log(msg.content);
+	 *     })
+	 *     .catch(err => console.log("couldn't get the message"));
+	 */
+
+	getMessage(channel, messageID, callback = (/*err, msg*/) => { }) {
+		return this.internal.getMessage(channel, messageID)
+			.then(dataCallback(callback), errorCallback(callback));
+	}
+
 	/**
 	 * Gets the banned users of a server (if the client has permission to)
 	 * @param {ServerResolvable} server server to get banned users of

src/Client/InternalClient.js

@@ -788,6 +788,34 @@ export default class InternalClient {
 		});
 	}
 
+	// def getMessage
+	getMessage(_channel, messageID) {
+		return this.resolver.resolveChannel(_channel)
+		.then(channel => {
+			if(!this.user.bot) {
+				return Promise.reject(new Error("Only OAuth bot accounts can use this function"));
+			}
+
+			if(!(channel instanceof TextChannel || channel instanceof PMChannel)) {
+				return Promise.reject(new Error("Provided channel is not a Text or PMChannel"));
+			}
+
+			var msg = channel.messages.get("id", messageID);
+			if(msg) {
+				return Promise.resolve(msg);
+			}
+
+			return this.apiRequest(
+				"get",
+				`${Endpoints.CHANNEL_MESSAGES(channel.id)}/${messageID}`,
+				true
+			)
+			.then(res => channel.messages.add(
+				new Message(res, channel, this.client)
+			));
+		})
+	}
+
 	// def getBans
 	getBans(server) {
 		server = this.resolver.resolveServer(server);