Allow flexible custom field placement

The order custom field merge codes can only be displayed in three locations: default, shipping, and billing. This is not always aesthetically pleasing especially when you have a custom merge code for international orders, then want a custom merge code asking for something like tax exempt forms. If we are able to place the custom field merge codes it would make our check out pages way more flexible and customizable to the stores needs and not forcing us to only use three locations.