add 3 optional config parameters

Not all exchanges are created durable, so the `exchange_durable` option allows you to specify this.

Also in some cases, you may only want to transmit the payload data over AMQP, especially since the tag is already the key. With the `payload_only` option, you will lose the original timestamp but have a simpler data structure.

Last, some AMQP Consumers will expect a content-type of application/json coming out of fluent, as this isn't streaming data. You can set this with the new `content_type`

Default behavior is the same for all three options.