Allow the OAuth client to receive a configuration block

[gaugau]

OAuth client can receive a block to config its internal http client (based on Faraday). https://github.com/intridea/oauth2/blob/0ab3c213f7baa43f232eb18fa46301e1a9460f7d/lib/oauth2/client.rb#L27 Hence, by allowing a block to our constructor, it's enable to change some helpful configurations such as log requests to STDOUT.