Added LTSV support and related configurations.

I have added LTSV(see http://ltsv.org) support to fluent-plugin-s3.

Change point is as below:
1. Added "output_data_type" config parameter to specify how data should be output. Available value: 'json'(default) / 'ltsv'.
Note: ltsv gem is required on using 'ltsv'
2. Added "include_tag_and_time" config parameter to specify whether or not time and tag should be output separately from the data section.
Available value: true(default) / false
3. Reimplemented "format_json" support: made it behave equivalently to specifying "output_data_type json" / "include_tag_and_time false"