Improve Shipping Zones so Post Codes can be configured

The current implementation of shipping zones doesn't work with UK postcodes. We would like the feature extended to allow rules to be set up based on UK postcodes. 

One way of implementing this would be to have a RegEx filter so that we can define a regex rule against a field (in this case Delivery Postcode) when defining the shipping zone.

The most common rules for UK postcodes would apply to the Postcode District (the combination of letters and numbers forming the first part of the postcode), and would allow filtering against multiple ranges, e.g. an area can be defined as AB36-38,AB55-56,FK17-21 etc.