Regex for validating UK phone numbers

Great work on this plugin, it's fantastic!

I have added basic support for validating UK phone numbers as well as US based on [this guide](http://aa-asterisk.org.uk/index.php/Regular_Expressions_for_Validating_and_Formatting_GB_Telephone_Numbers#Match_GB_telephone_number_in_any_format) if you would like to consider pulling in.

Cheers