Add support for capturing access token values

Vendors (such as Google) sometimes pass additional information with
access token responses. Capturing these values allows package users to
make use of them when appropriate.

Refs thephpleague/oauth2-google#14

Allow using a proxy for debugging.

Add zendesk provider to readme

Add Betaseries provider

Add Untappd provider

https://untappd.com/api/docs#authentication

Add Drupal OAuth2-Server provider

Update provider guide to refer to authentication traits

Add MailChimp provider

Add documentation around authentication headers in custom provider class

Update README.PROVIDERS.md

Add link to OAuth2 PSR7 Middleware library

Add Azure Active Directory provider

The provider is already listed on the official site (http://oauth2-client.thephpleague.com/providers/thirdparty/) just missing here.

Added the Ring Central provider

Add Fitbit provider to readme

Enable dynamic parameters being passed into the authorization URL

As per - https://github.com/thephpleague/oauth2-client/pull/485 - adding tests to handle custom authorization URL parameters.

When for example using Azure AD, you may need to pass additional parameters to the authorization URL - like `prompt`. This change enables passing custom parameters to the authorization URL as per each provider's needs. More options for Azure AD can be found [here](https://msdn.microsoft.com/en-us/library/azure/dn645542.aspx)
For Google those are for example access_type or login hint - found [here](https://developers.google.com/youtube/v3/guides/auth/server-side-web-apps)

Add Eve Online provider

Fix resource_owner_id being put into the wrong array

Include resource owner id in access token json

Fixes #472

Add tests for array accessor trait, fixed one bug