Add return constants package, and common message sender functions

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

lib/appcontext/exported.go

@@ -1,12 +1,8 @@
 // i2_bot – Instinct PokememBro Bot
-// Copyright (c) 2017 Vladimir "fat0troll" Hodakov
+// Copyright (c) 2017-2018 Vladimir "fat0troll" Hodakov
 
 package appcontext
 
-var (
-	a *Context
-)
-
 // New is a Context creation function
 func New() *Context {
 	c := &Context{}