Add currency to config

[veeqtoh]

The default Cashier currency is Nigeria Naira (NGN) and can be changed by calling the Cashier::useCurrency method from within the boot method of one of your service providers. The useCurrency method accepts two string parameters: the currency and the currency's symbol:

use Veeqtoh\Cashier\Cashier;

Cashier::useCurrency('ngn', '₦');
Cashier::useCurrency('ghs', 'GH₵');

We need to make this more flexible by adding this settings to the config/env.

Remember to update documentation.