Add command middleware function functionality

That `Command#use` doc is ridiculous. I will definitely need to write a detailed help page for it and outsource that info there eventually.

src/lib/command/Command.js

@@ -175,6 +175,9 @@ export default class Command
 		 */
 		this.overloads = info.overloads || null;
 
+		// Middleware function storage for the Command instance
+		this._middleware = [];
+
 		if (this.overloads && this.group !== 'base') throw new Error('Commands may only overload commands in group "base"');
 
 		// Default guildOnly to true if permissions/roles are given
@@ -217,6 +220,44 @@ export default class Command
 		if (!(this.action instanceof Function)) throw new Error(`Command#${name}.action: expected Function, got: ${typeof this.action}`);
 	}
 
+	/**
+	 * Adds a middleware function to be used when the command is run
+	 * to make modifications to args or determine if the command can
+	 * be run. Takes a function that will receive the message object
+	 * and the array of args.
+	 *
+	 * A middleware function must return an array where the first item
+	 * is the message object and the second item is the args array.
+	 * If a middleware function returns a string, or throws a string/error,
+	 * it will be sent to the calling channel as a message and the command
+	 * execution will be aborted. If a middleware function does not return
+	 * anything or returns something other than an array or string, it will
+	 * fail silently.
+	 *
+	 * Example:
+	 * ```js
+	 * this.use((message, args) => [message, args.map(a => a.toUpperCase())]);
+	 * ```
+	 * This will add a middleware function to the command that will attempt
+	 * to transform all args to uppercase. This will of course fail if any
+	 * of the args are not a string. This could be ensured with the `stringArgs`
+	 * `argOpts` flag, or via another middleware function added before this one
+	 * that converts all args to strings.
+	 *
+	 * Note: Middleware functions should only be added to a command one time each,
+	 * and thus should be added in the Command's constructor. Multiple middleware
+	 * functions can be added to a command via multiple calls to this method
+	 * @memberof Command
+	 * @instance
+	 * @param {Function} fn - Middleware function. `(message, args) => [message, args]`
+	 * @returns {Command} This command instance
+	 */
+	use(fn)
+	{
+		this._middleware.push(fn);
+		return this;
+	}
+
 	// Send provided response text to the command's calling channel
 	// via edit, editCode, sendMessage, or sendCode depending on Whether
 	// or not the bot is a selfbot and/or a codeblock language is given

src/lib/command/CommandDispatcher.js

@@ -60,7 +60,29 @@ export default class CommandDispatcher
 		if (!this.hasRoles(dm, message, command)) return this.missingRolesError(message, command);
 		if (guildStorage) message.guild.storage = guildStorage;
 
-		await this.dispatch(command, message, args, mentions, original).catch(console.error);
+		let middlewarePassed = true;
+		for (const middleware of command._middleware)
+			try
+			{
+				let result = middleware(message, args);
+				if (!(result instanceof Array))
+				{
+					if (typeof result === 'string') message.channel.send(result);
+					middlewarePassed = false;
+					break;
+				}
+				[message, args] = result;
+			}
+			catch (err)
+			{
+				middlewarePassed = false;
+				message.channel.send(err.toString());
+				break;
+			}
+
+		if (middlewarePassed)
+			await this.dispatch(command, message, args, mentions, original).catch(console.error);
+
 		const dispatchEnd = now() - dispatchStart;
 
 		/**
@@ -147,7 +169,7 @@ export default class CommandDispatcher
 			.map(a => typeof a === 'string' ? a.trim() : a)
 			.filter(a => a !== '' && !(typeof a !== 'string' && isNaN(a)));
 
-		return { command: command, mentions: mentions, args: args, content: content, dm: dm };
+		return { command, mentions, args, content, dm };
 	}
 
 	/**