proxy mode

I would like to use gcp-iap-auth as a proxy in front of other webapps: [iap] -> [gcp-iap-auth] -> [backend app]. Assuming this will work. I started working on a small utility to do this and realized I was importing so much of gcp-iap-auth that maybe it would make sense to implement proxy-mode as an option, perhaps a flag like -proxy http://localhost:8080.

I am happy to send a PR, time permitting, if you think this may be acceptable?