user-configured redirect URL clobbered in oauthproxy.go

see line 55 of oauthproxy.go ([link](https://github.com/bitly/oauth2_proxy/blob/a94b0a8b25e553f7333f7b84aeb89d9d18ec259b/oauthproxy.go#L155))

We literally overwrite whatever the value is with `fmt.Sprintf("%s/callback", opts.ProxyPrefix)` even if the user provided a redirect URL they want to use.