Allow Config implementation to be specified

[hassankhan]

Expected Behavior

$opts = ['config' => MyConfig::class];
$client = new Instagram($clientId, $clientSecret, $accessToken, $redirectUrl, $opts);

Possible Solution

Add a new option to the configuration options. Use the option in a provider to instantiate it.

Context

Lets developers use a class rather than passing an array of parameters to the constructor.