Added possibility to ignore JSON format and added backslashes escape for JSON

1) Now it is possible to send logs in plain text format when nginx fluentd_tag is empty: (fluentd_tag '';)
2) curl 'localhost/\t' produced broken JSON forma and td-agent was not able to parse JSON format. I've added possibility to escape backslashes if necessary.

First of all I've tried to implement this feature without additional escaped_line variable, but I was not able to determine how many slashes are there to alloc the extra memory. So I had to create new variable.