Support for 'channel' data type

Following Logstash's documentation (https://www.elastic.co/guide/en/logstash/current/plugins-inputs-redis.html), add support for the `channel` data type. Add the following option:

```
redislog.data_type = 'DATA_TYPE'
```

Where `DATA_TYPE` is one of the following two values:
- `list` (default, supporting the `RPUSH` command)
- `channel` (supporting the `PUBLISH` command)
