Implement plugin system (#1020)

* Implement basic plugin system

* The dawn of new era

* Add plugins warning

* Move more members to PublicAPI

* Open commands for the plugins

* Add IBotHackNewChat

* Run plugin events in parallel

* Use properties in IPlugin

* Hook our custom plugin into CI to ensure it compiles

* Fix dotnet brain damage

* Add IBotsComparer

* Add code documentation

* Add IBotTradeOffer

* Add IBotTradeOffer example

* Add IBotTradeOfferResults

* Final bulletproofing

* Final renaming

ArchiSteamFarm/Plugins/IBotCommand.cs

@@ -0,0 +1,39 @@
+//     _                _      _  ____   _                           _____
+//    / \    _ __  ___ | |__  (_)/ ___| | |_  ___   __ _  _ __ ___  |  ___|__ _  _ __  _ __ ___
+//   / _ \  | '__|/ __|| '_ \ | |\___ \ | __|/ _ \ / _` || '_ ` _ \ | |_  / _` || '__|| '_ ` _ \
+//  / ___ \ | |  | (__ | | | || | ___) || |_|  __/| (_| || | | | | ||  _|| (_| || |   | | | | | |
+// /_/   \_\|_|   \___||_| |_||_||____/  \__|\___| \__,_||_| |_| |_||_|   \__,_||_|   |_| |_| |_|
+// 
+// Copyright 2015-2019 Łukasz "JustArchi" Domeradzki
+// Contact: JustArchi@JustArchi.net
+// 
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// 
+// http://www.apache.org/licenses/LICENSE-2.0
+// 
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using System.Threading.Tasks;
+using JetBrains.Annotations;
+
+namespace ArchiSteamFarm.Plugins {
+	[PublicAPI]
+	public interface IBotCommand : IPlugin {
+		/// <summary>
+		///     ASF will call this method for unrecognized commands.
+		/// </summary>
+		/// <param name="bot">Bot object related to this callback.</param>
+		/// <param name="steamID">64-bit long unsigned integer of steamID executing the command.</param>
+		/// <param name="message">Command message in its raw format, stripped of <see cref="GlobalConfig.CommandPrefix" />.</param>
+		/// <param name="args">Pre-parsed message using standard ASF delimiters.</param>
+		/// <returns>Response to the command, or null/empty (as the task value) if the command isn't handled by this plugin.</returns>
+		[NotNull]
+		Task<string> OnBotCommand([NotNull] Bot bot, ulong steamID, [NotNull] string message, [NotNull] string[] args);
+	}
+}