mailru: add Mail.Ru OAuth2 endpoint
mailru: add Mail.Ru OAuth2 endpoint

Mail.Ru is one of the most popular email service provider in Russia.

The documentation for Mail.Ru's OAuth2 endpoints is at
https://o2.mail.ru/docs/

Change-Id: I605979b85cff2b00c7b65bbd3306d3fa66faa56d
Reviewed-on: https://go-review.googlesource.com/84415
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/oauth2