Add support for commands monitoring events-only

[bmeneg]

Correcting the last PR closed: no changes were made on callback interface, thus not breaking Slack/Telegram calls.

This allow implementation of commands/plugins that relies on JOIN events like the "greetings" command that we were thinking about.

A simple example of using this feature would be like the following one:

package greetings

import (
	"fmt"
	"github.com/meleca/bot"
)

func SayHello(cmd *bot.EventCmd) (string, error) {
	if cmd.EventCode == "JOIN" {
		message := "hello " + cmd.User.Nick
		return message, nil
	}
	return "", nil
}

func init() {
	bot.RegisterEventCommand(
		"greetings",
		SayHello)
}

If you guys think something could be done in a better way, please let me know.

PS.: this PR solves issue #1