Allow custom oauth_authenticator

[Nate Harmon]

The AuthManager API provides the option to initialize with an oauth_authenticator object. However, this object is not currently passed down to the init_loaders method akin to the login_authenticator object.

Changing lines 199-200 in keg_auth.core.init_loaders from:

        if app.config.get('KEGAUTH_OAUTH_PROFILES'):
            self.oauth_authenticator = OAuthAuthenticator(app)

to

        if app.config.get('KEGAUTH_OAUTH_PROFILES'):
            self.oauth_authenticator = self.oauth_authenticator_cls(app)

would allow the ability to create an AuthManager with a customized oauth_authenticator object.