Raise "started" event analogous to the "stopped" event

[tobilg]

Background

We have some issues when creating integration tests, mostly during setup and cleanup. The docs at https://bbc.github.io/sqs-consumer/interfaces/Events.html state that there's an stopped event, but the equivalent started event is missing unfortunately. Would it be possible to add this?

Objectives

1. Add a new started event

[nicholasgriffintn]

Thanks for the suggestion, I've created a ticket here: #388