Adjust title of "Shipping Address" to "Mailing Address"

[andrewlimaza]

Is your feature request related to a problem? Please describe.
The term "Shipping Address" may be confusing to customer's as to what it means. An updated name for this would be "Mailing Address" and we should cater for this terminology by default.

Describe the solution you'd like
Add in an option (textfield) to allow site owner's to easily change or set the title of the fields to anything they would like and give them the flexibility. This would probably be best to be included in the Advanced Settings area of PMPro.

Summary: A function that pulls in the option value from the database or default value if that option is empty, a filter before returning the value and a setting in the Advanced Settings to tie it all together. Use this method wherever we set the fields title.

Describe alternatives you've considered
A temporary solution is to use the gettext filter to replace the wording now. Updating it to use the above solution will make it slightly more difficult to translate - the idea would be to remove the label option completely to set the default label and let plugins translate this string.