Question / Request: add full_message support for legacyFormat = false

[smantel-ch]

Hi
First of all, thanks for creating this transporter!
I also would like to outline that I only recently started using winston and winston-log2gelf.

While playing around with this transporter I realised that full_message is set as info.message:
https://github.com/Buzut/winston-log2gelf/blob/master/index.js#L195
fullMessage = info.message;

I'm not familiar what exactly legacyFormat is but when setting it to true I can pass metadata and it is added as JSON to full_path

Would it make sense to do something like this when legacyFormat is false:
fullMessage = (info.full_message !== '' || info.full_message === undefined) ? info.full_message : info.message;
And yes, this would mean the meta would need to have a property called full_message