Custom formatting options and updated devDependencies

Hey there! Stumbled across this module today and loved the simplicity of the middleware compared to many others. I decided to take it upon myself to extend this a little bit and offer custom tokens to be passed in to have better control over the output that is sent to Winston.

I've updated the `README.md` as well as (re-)written tests, as some of the dependencies were very old. Would love to get this merged in!

Let me know if you have any questions/concerns.

Evan