Line feed extension

Hi, @tagomoris -san.

This is a proposal. Support "\n" to send multiple lined messages to ikachan.

The reason why I needed this extension is basically because I wanted to send multiple lined messages which is outputted by my fluent-plugin-grepcounter, https://github.com/sonots/fluent-plugin-grepcounter (`output_matched_message true` option). 

This extension should be useful for other users too because this enables to write "\n" in config file as fluent-plugin-mail already supports https://github.com/u-ichi/fluent-plugin-mail/pull/1/files. 
This extension should give more flexibilities to the message fields.