Add return constants package, and common message sender functions

Trying to resolve as much as possible ``gometalinter`` issues.

lib/sender/senderinterface/senderinterface.go

@@ -0,0 +1,16 @@
+// i2_bot – Instinct PokememBro Bot
+// Copyright (c) 2018 Vladimir "fat0troll" Hodakov
+
+package senderinterface
+
+import (
+	"github.com/go-telegram-bot-api/telegram-bot-api"
+)
+
+// SenderInterface implements Sender for importing via appcontext
+type SenderInterface interface {
+	Init()
+
+	SendMarkdownAnswer(update *tgbotapi.Update, message string)
+	SendMarkdownMessageToChatID(chatID int64, message string)
+}