Support url prefix

[tadatuta]

• https://github.com/bem/bem-forum/blob/master/server/auth.js#L36
• https://ru.bem.info/forum/login_callback/?error=redirect_uri_mismatch&error_description=The+redirect_uri+MUST+match+the+registered+callback+URL+for+this+application.&error_uri=https%3A%2F%2Fdeveloper.github.com%2Fv3%2Foauth%2F%23redirect-uri-mismatch
• https://github.com/bem/bem-forum/blob/master/server/router.js#L35