Expose custom agent parameter?
Since `purest` [allows for it](https://github.com/simov/purest/issues/14), I would love to get the same support for `grant`. That would allow me to control the whole oauth + user profile fetching flow.

Namely, I want to use this with a proxy agent like: https://github.com/TooTallNate/node-proxy-agent

This is necessary as nodejs core still haven't expose an overridable `http.globalAgent` api yet, sorry for nagging you on this: https://github.com/nodejs/node/issues/23281