Allow using logstash option with the File transport

Hello,

I noticed a nice logstash format option by browsing your code, and after reading the last PR I saw you're only using it with another library.
Logging in a file using the logstash format is useful if you use a centralized logstash installation, as we do.

This PR only allows people to add a logstash parameter to format the output, and take advantage of the already written code.

I'm not sure it's useful for the other transports though, so I only added that for the File transport.