add default column option

[giwa]

default_timezone: If input column type (embulk type) is timestamp, this plugin needs to format the timestamp into a SQL string. This default_timezone option is used to control the timezone. You can overwrite timezone for each columns using column_options option. (string, default: UTC)

 default_column_options:
    TIMESTAMP: { type: string, timestamp_format: "%Y/%m/%d %H:%M:%S", timezone: "+0900"}
    BIGINT: { type: string }

https://github.com/embulk/embulk-output-jdbc/tree/master/embulk-output-mysql