Add config options: wait, include_time

```ruby
    config_param :wait,   :bool,    :default => false
```

When set to `true`, the plugin will retry flushing the buffer if there were no subscribers at the other end of redis. It will _not_ retry if it has already published at least one record within the flush operation.

This is to conform to fluentd's at-most-once delivery policy while trying to prevent data loss as much as possible: seeing that there were no easy way to cancel the flush mid-way in the `msgpack_each` loop, I resorted to checking the subscriber count only at the beginning of the flush.

```ruby
    config_param :include_time, :bool,  :default => true
```

Setting this to `false` makes the plugin _not_ to include the `time` value in the record published to redis. This is useful when timestamping is already handled upstream.