Add support for custom schemes

[monolithed]

Please add support for custom schemes without client_secret.

import OAuthManager from 'react-native-oauth';

let manager = new OAuthManager('Foo');

manager.addProvider({
   foo: { client_id: 'foo' }
});

manager.configure({
	auth_version: '2.0',
	authorize_url: 'foo://login',
	scopes: 'userinfo',

	callback_url: ({ app_name, client_id }) => {
		return 'foo-oauth-callback://';
	}
});

Maybe duplicate of https://github.com/fullstackreact/react-native-oauth/issues/176 https://github.com/fullstackreact/react-native-oauth/issues/128