Add extract_access_token option to OAuth2::Client

Created by: jonspalmer

Make OAuth2::Client responsible for parsing the access_token from the token response by adding an extract_access_token option. This enables clients to supply their own extraction logic so that we can support various use cases like Pintrest and Microsoft that have their own particular wrinkles.

This is a combination of ideas #516 and #517 but based of the 1-4-stable branch and in a way that is a non-breaking change.

There will be a little bit of work to merge this into master but there is a clear path there too.