Add encode_msgpack_hex option

Currently, I am using Hadoop Streaming with Amazon Elastic MapReduce. overhead of encode / decode the json is the bottleneck. So, I wanted a version that you want to output in binary format of msgpack that can be handled much faster.

It was mounted on the fluent-plugin-s3, in order to be able to be processed immediately without running the program the conversion process separately at the time the log is uploaded to s3.

You are output in hexadecimal, and this is because it is easier to read line by line from STDIN. I want to adopt there if there is a good format to another.