adding splat() functionality

[vicortez]

Hi, the library is very good, but it seems to not support logs like

console.log("the value is %j",value )

winston can support this with the splat() formatter, but your library is lacking that. Do you think it would make a good feature to add?

[Seigneurhol]

I forked the repo and solve this issue here : https://github.com/Seigneurhol/discord-winston-transport