Add fallback logic to address flags (closes #32)

[bfiessinger]

implements flag fallbacks for Addresses as described in #32

How does it work?

using the magic __call method we are searching the flags defined in config and eventually use them to retrieve the addresses.
We can still call methods like getShippingAddress and getPrimaryAddress, but instead of using a dedicated method for this we now can utilize the getAddressByFlag method which walks recursively through the flags array defined in the config.

So if the flags array looks like this:

'flags' => ['public', 'primary', 'billing', 'shipping'],

and we are calling getBillingAddress the new implementation would walk up the array until it reaches the public flag if it cannot find any addresses earlier. It would not check for the shipping flag in this step.

After trying all flags and if there is still no address found based on the direction parameter the new implementation would either return the first or the last attached address as a last fallback step.

This ensures to return an address object in almost every situation.