twitch: add twitch OAuth2 endpoint
twitch: add twitch OAuth2 endpoint

Twitch Auth API documentation: https://dev.twitch.tv/docs/authentication

Change-Id: I1a517609c229c8893d2dbf91f8821db386f822ab
Reviewed-on: https://go-review.googlesource.com/83136
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/oauth2