Reader: consume from multiple topics

[alebabai]

Describe the solution you'd like
Please, add opportunity to set multiple topics into reader to fetch from.

Supporting documentation
For now it supports single topic and pass it into consumerGroup (if GroupID is specified) as a string slice with one element

kafka-go/reader.go

Lines 656 to 663 in 2b3f682

	if r.useConsumerGroup() {
	r.done = make(chan struct{})
	r.runError = make(chan error)
	cg, err := NewConsumerGroup(ConsumerGroupConfig{
	ID: r.config.GroupID,
	Brokers: r.config.Brokers,
	Dialer: r.config.Dialer,
	Topics: []string{r.config.Topic},

Related
#564

[dominicbarnes]

This improvement was recently released with v0.4.12