Add NewAmqpCosnumer func

[cyberfined]

I've added new function to amqp package called NewAmqpConsumer. It's a replacement for StartAmqpConsumer that has one problem - it never returns a value, because it calls cosumer.Run method that starts an infinite loop of message processing, so graceful shutdown is impossible for such a consumer.

[cyberfined]

Example:

var consumer *rabbitmq.Consumer
go func() {
    consumer = amqp.StartAmqpConsumer(conn, handler, key, queue)
}

// Some code here

consumer.Close()

Traceback:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x60 pc=0xf5048b2]

goroutine 9 [running]:
github.com/wagslane/go-rabbitmq.(*Consumer).Close(0x0?)
	/Users/user/go/pkg/mod/github.com/wagslane/go-rabbitmq@v0.14.2/consume.go:125 +0x12

It happens because consumer always equals to nil