Delete consumer group offset

[faillefer]

Description

Kafka 2.4.0 introduced a new api to delete consumer offsets ( KIP-496: Administrative API to delete consumer offsets )
The API has been implemeted in sarama (IBM/sarama#2006).

This PR add support for this operation in kafkactl through a new command delete consumer-group-offset.
The expected usage is :

Usage:
  kafkactl delete consumer-group-offset CONSUMER-GROUP --topic=TOPIC --partition=PARTITION [flags]

Aliases:
  consumer-group-offset, cgo, offset

Flags:
  -h, --help              help for consumer-group-offset
  -p, --partition int32   delete offset for this partition. -1 stands for all partitions (default -1)
  -t, --topic string      delete offset for this topic

topic flag is required.

This MR requires a update of sarama version to work.

Fixes #88

Type of change

• New feature (non-breaking change which adds functionality)

Documentation

• the change is mentioned in the ## [Unreleased] section of CHANGELOG.md
• the configuration yaml was changed and the example config in README.md was updated
• a usage example was added to README.md

[d-rk]

@faillefer I tested it with sarama 1.30.0 and it worked fine.
I locally rebased, updated sarama and merged into master. So it is already merged, but not showing up in this PR.
I will just close it.

Thanks again for the contribution!